Atom a Closer Look

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What Is This Module About?

Can you recall the definition of matter? Matter is anything that has mass and occupiesspace. Therefore, everything around you is matter. Each type of matter has a definite set ofproperties. Water is colorless, alcohol easily evaporates and copper can be hammered into thinsheets. The characteristics of different types of matter help you distinguish one from another.

Suppose you have a teaspoon of sugar. How does it taste? Definitely, it tastes sweet. If yougrind the sugar into very fine powder and taste it again, will it still be sweet? Yes, the fine sugarpowder will still be sweet. But what is the smallest particle of sugar you can come up with thatwill still taste sweet?

If you look at a substance, it appears to be composed of matter that is visible to your eyes.But if you could divide this substance into smaller and smaller particles, you will finally come upwith the atoms of that substance. Atoms are called the building blocks of matter. All substancesare made up of these very tiny particles.

In this module you will learn more about atoms. You will find out how these atoms are madeup of even smaller particles. You will also discover how scientists came up with theories toexplain the nature of the atom and the subatomic particles.

There are three lessons in this module. These are:

Lesson 1 – The History of the Atom

Lesson 2 – The Subatomic Particles

Lesson 3 – Locating the Electron

Wait!

Before you read this module, make sure you have already read Matter 1A and IB: Forms,Properties and Changes.

What Will You Learn From This Module?

After studying this module, you should be able to:

♦ cite the contributions of the early scientists to the development of theatomic theory;

♦ identify and describe the subatomic particles;

♦ draw a general picture of the structure of the atom;

♦ determine the number of protons, neutrons and electrons in an atom;

♦ identify and differentiate the isotopes of an element;

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♦ identify the atomic orbitals present in each energy level of the atom; and

♦ give the number of electrons in each energy level.

Let’s See What You Already Know

Before you continue reading this module, find out first how well you know the topics to bediscussed by taking the following test. Write the letter of the correct answer on the line beforeeach number.

_______ 1. All atoms of the same element are alike. This idea was proposed by ____ .

a. Democritusb. Leucippusc. Rutherfordd. Dalton

_______ 2. According to Democritus, all forms of matter are composed of tiny particlescalled _____ .

a. electronsb. protonsc. atomsd. neutrons

_______ 3. The nucleus is composed of _____ .

a. protons and electronsb. protons and neutronsc. electrons and neutronsd. protons, electrons and neutrons

_______ 4. The atomic number gives the number of _____ in an atom.

a. electronsb. electrons and protonsc. neutronsd. neutrons and protons

_______ 5. The mass number of an atom indicates the number of _____ in an atom.

a. neutronsb. protonsc. electrons and protonsd. protons and neutrons

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_______ 6. All of the following characterize a proton except:

a. It is positively charged.b. It is found in the nucleus.c. It has a mass equal to that of the neutron.d. It has no charge.

_______ 7. If an element is described by these data: Z = 17 and A = 35, which of thefollowing statements is true of the element?

a. It has 17 protons and 17 neutrons.b. It has 17 protons and 18 neutrons.c. It has 17 electrons and 18 protons.d. It has 17 electrons and 17 neutrons.

_______ 8. An electron in the first energy level may be found in a/an _____ .

a. s orbitalb. s or p orbitalc. d orbitald. p orbital

_______ 9. An atom has an atomic number of 11 and a mass number of 23. Which ofthe following statements is correct?

a. The atom has 11 electrons and 11 neutrons.b. The atom has 11 electrons and 11 protons.c. The atom has 11 electrons and 12 neutrons.d. The atom has 12 electrons and 11 neutrons.

_______10. For the isotopes 8O16 and

8O17, which statement is not true?

a. There are 8 electrons in both isotopes.b. There are 8 neutrons in both isotopes.c. The number of protons in both isotopes is 8.d. O16 has 8 neutrons while O17 has 9 neutrons.

Well, how was it? Do you think you fared well? Compare your answers with those in theAnswer Key on page 28 to find out.

If all your answers are correct, very good. This shows that you already know much aboutthe topics in this module. You may still study the module to review what you already know. Whoknows, you might learn a few more new things as well.

If you got a low score, don’t feel bad. It means that this module is for you. It will help youunderstand some important concepts that you can apply in your daily life. If you study this modulecarefully, you will learn the answers to all the items in the test and a lot more! Are you ready?

You may go to the next page to begin Lesson 1.

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LESSON 1

The History of the Atom

A lot of people now take for granted that matter is made up of molecules, which in turnare made up of atoms. There are instruments now that enable us to look at these tiny particles ofmatter. But have you ever thought about what it was like for people who lived in ancient timeswho did not have powerful optical instruments at their disposal for studying tiny materials? Whatdid they think of matter? Did they see it the way we do now, as being made up of a seeminglyinfinite number of tiny particles? How did the idea of the atom come about? These are the thingsyou will find out in this lesson. In this lesson, you will find out how our present knowledge of theatom came to be. The atom has a really very interesting history that dates back to ancient times.Would you like to know about it? Read on!

Let’s Try This

Before you proceed to learn the history of the atom, do the following activity first tounderstand the concept of the atom.

1. Get a pile of paper clips, all of the same color and size.

2. Divide the pile into two equal piles.

3. Divide each pile into two smaller equal piles.

4. Repeat step 3 until you are down to a pile containing only one paper clip. Can thispaper clip still do the “job” of a paper clip, that is, can it hold loose sheets of papertogether? Of course it can!

5. Take a pair of scissors and cut that one paper clip in half. Can half of the paper clip dothe same job as the single paper clip has?

If you do the same thing with any element, you will reach an indivisible part that has thesame properties of the element, like the single paper clip. This indivisible part is called the atom.

Let’s Learn

Scientific ideas first came about in ancient Greece. Do you know where Greece is? It is asmall country in Europe, which is considered the cradle of civilization because it is where art,philosophy and science first flourished.

Ancient Greek philosophers were the first to speculate on the structure of matter. TheseGreek philosophers were definitely different from modern-day scientists. They certainly did nothave the scientific method to go by and neither were they patient and industrious enough to

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conduct experiments. Their ideas were all based on speculation on the things they saw aroundthem. No wonder, their ideas are so different from what we know now!

Empedocles (circa 440 B.C.), for instance, proposed that matter was made up of fourelements—fire, earth, water and air. What do you think led him to such a conclusion? Well,maybe he saw a log of green wood burning. He saw the flame (fire) emerge, as well as the sap(water) and smoke (air). What remained was the ashes (earth).

At around the same time, another philosopher, Leucippus, proposed an entirely differentidea. According to Leucippus, matter was made up of particles that were so tiny they could notbe divided into smaller units. He and his pupil, Democritus, were able to come to this conclusionthrough speculation and observation. They saw how a stream of water could be broken intodrops and how the apparently solid beach could be seen and felt as grains of sand. This madethem come up with the concept of an indivisible unit of matter. In fact, the word atom comesfrom the Greek words that mean “not to cut.” The term is attributed to Democritus.

Democritus

water

air

earth

fire

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Almost a century later, Aristotle (384–322 B.C.), the famous Greek philosopher, declaredthat Leucippus and Democritus were wrong, and Empedocles was right. He was supported byPlato. Because these two were the most influential thinkers of their time, the concept of indivisibleunits of matter was abandoned and forgotten for centuries.

It took two thousand years before the concept of the atom was revived. The oneresponsible for this was an Englishman named John Dalton (1766–1844). Dalton conductedseveral experiments on the weights of substances involved in chemical reactions. The data heobtained made him conclude that the ultimate particles of homogeneous matter (elements) areperfectly alike in weight and other properties.

Democritus is wrong. Iagree with whatEmpedocles said aboutmatter.

I agree with Aristotle.There is no such thingas an indivisible unit ofmatter.

Based on myexperiements, Iconclude that thesmallest particles ofall substances areperfectly alike inweight and otherproperties. Theseparticles are called“atoms.”

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In 1804, Dalton proposed the atomic theory. According to this theory,

Dalton’s theory gained wide acceptance among scientists. It opened the door to a largeamount of chemical investigation. Elements were discovered and named, and the periodic table ofelements was formulated.

Atoms combine with other atoms toform compounds but remain unchangedduring ordinary chemical reactions.

Gold bar Iron metal

Au Au

Au Au

Fe Fe

Fe Fe

Atoms can combine in small ratios ofwhole numbers such as 1:1, 2:1, 2:3,etc.

one atomof carbon

four atoms ofhydrogen

CH H

H H

H

H

H

C

H

Matter is made up of extremely smallindestructible particles called atoms.

Goldring Gold atoms

Au Au Au

Au Au Au

All atoms of a given element are alike.

Gold atomsGold ring

Gold atoms

Au Au Au

Au Au Au

Au Au Au

Au Au AuGold bar

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Let’s Think About This

There are two important laws of chemical combination that were explained by Dalton’satomic theory. These laws are the law of conservation of mass and the law of definiteproportions. Below are the definitions of the two laws. Below each definition, write the postulateof the atomic theory that serves to explain the law.

1. The law of conservation of mass states that in an ordinary chemical reaction, there isno detectable change in the masses of the substances involved before and after thereaction.

________________________________________________________________

_____________________________________________________________

2. The law of definite proportions states that a given chemical compound always showsa fixed proportion. A compound always contains the same elements in the samepercentages by mass.

________________________________________________________________

_____________________________________________________________

Are your answers the same as these?

1. The law of conservation of mass can be explained by the third postulate of Dalton’satomic theory. According to this postulate, when atoms combine with other atoms toform compounds, they remain unchanged. Since the same atoms are present beforeand after a chemical reaction, the masses of the reactants and products must be thesame as well.

2. The law of definite proportions is explained by the second and fourth postulates of theatomic theory. According to the second postulate, all atoms of the same element arealike; according to the fourth postulate, atoms combine in fixed ratios. This means thatthat the percentage composition of a compound is always the same regardless of itsorigin and how it was prepared.

Let’s Learn

When Dalton proposed the atomic theory, a lot of questions about the nature of matter wereresolved. The existence of the atom was readily accepted by scientists. But the question of howthe atom looked was not quite as easy to agree on. The knowledge of the structure of the atomcame to be because of studies on the electrical nature of matter.

Discoveries on the nature of electricity led scientists to suspect that there is a relationshipbetween electricity and the structure of matter. However, the atomic theory did not give anexplanation of this phenomenon.

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Much of the understanding of the atom came from studies on electric discharges throughgases, specifically the cathode rays. Cathode rays are some kind of matter that are emitted fromthe negative electrode (cathode) in a gas tube. These rays made the positive end of the tube(anode) emit a greenish light.

Numerous experiments showed that the cathode rays have the following properties:

♦ Cathode rays are emitted from the negative end of a vacuum tube when electric currentis passed through the tube.

♦ They travel in a straight line.

♦ The cathode rays cause glass and other materials to fluoresce or glow.

♦ The cathode rays bear a negative electric charge. This is shown by their attractiontoward positively charged objects held outside the vacuum tube.

The studies on cathode rays led Sir Joseph John Thomson (1856–1940), an Englishphysicist, to conclude that cathode rays are negatively charged fundamental particles found in allatoms. These cathode rays are now known as electrons.

At the same time that studies were being made on cathode rays, Eugen Goldstein (1850–1930), a German physicist, discovered the existence of positive particles which he called canalrays. These particles are attracted to the cathode and are massive. They are now known asprotons.

With these discoveries, Dalton’s postulate that the atom was indestructible was proven to beuntrue. The atom is made up of even smaller particles!

Let’s Review

Differentiate cathode rays from canal rays. How did the existence of these two particlesviolate the first postulate of the atomic theory?

____________________________________________________________________

________________________________________________________________________

_____________________________________________________________________

Compare your answers with those in the Answer Key on page 28.

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Let’s Learn

With the discovery of the existence of smaller particles inside the atom, what do you thinkwas the next thing that scientists tried to figure out? How these particles are arranged inside theatom, of course!

The first one to propose a model for the structure of the atom was J.J.Thomson who, as youknow, was the one who discovered the electron. Thomson offered a model that is called theraisin-bread model of the atom. Do you know what raisin bread looks like? Draw a picture of aslice of raisin bread in the space below.

Compare your picture of the raisin bread slice with Thomson’s model of the atom below.

Do you now see how they are alike?

Thomson’s model of the atom

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According to Thomson, the atom was composed of a positively charged sphere withelectrons loosely embedded in the surface.

Ernest Rutherford (1871–1937), another British physicist, tested Thomson’s model of theatom. In his experiment, he used a beam of alpha rays (positively charged particles) to direct at asheet of gold foil. These alpha rays were to be emitted by a piece of radium, a radioactiveelement. He presumed that if Thomson’s model was correct, the alpha rays would pass throughthe foil with very slight deflections because the positive charge of the atom was diffused throughthe atom.

The following is the setup of the experiment:

Rutherford recorded the following observations:

1. Most of the alpha particles passed through the foil without deflections.

2. A few alpha particles bounced back with large angles of deflection.

3. A few passed through with large angles of deflection.

These results amazed Rutherford. They were totally unexpected. Based on the results of thisexperiment, Rutherford came up with the nuclear model of the atom. In fact, he won a NobelPrize for his discovery of the nucleus. According to Rutherford’s model, the atom is mostly emptyspace with a tiny, positive central core called the nucleus. This is where most of the mass of theatom is concentrated. Moving around the nucleus are the electrons, which are so light in weight.Below is a picture of Rutherford’s atom.

Rutherford’s model of the atom

Thin gold foilFluorescent

screen

Radium Leadshield

A few weredeflected atlarge angles.

Most particlespassed throughin a straight line.

A few particlesbounced back.

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Can you now explain the results of Rutherford’s experiment in terms of his model of theatom? Write your answer in the space below.

____________________________________________________________________

________________________________________________________________________

________________________________________________________________________

____________________________________________________________________

Compare your answer with mine:

According to Rutherford’s model, the atom is mostly empty space with a tiny positivecore—the nucleus. Most of the alpha particles passed through the gold foil without deflection.This meant that they passed through the empty space of the gold atoms. Some alpha particlesbounced back with deflections. This meant that they hit the positive nucleus. Since alpha rays arepositively charged and like charges repel each other, they were repelled by the nucleus. Somealpha rays passed through with slight deflection. This meant that they hit the electrons, which areso light in weight that they did not cause the alpha rays to bounce at large angles.

Let’s See What You Have Learned

A. Identify who/what is being described in each of the following statements. Write youranswer on the line before the number.

________________ 1. He based his theory on large amounts of experimental dataobtained from the weights of substances involved inchemical reactions.

________________ 2. They were later known as electrons.

________________ 3. He proposed the raisin-bread model of the electron.

________________ 4. He concluded that matter is made up of water, fire, earthand air.

________________ 5. According to this theory, the sum of the masses of thereactants in a chemical reaction is equal to the sum of themasses of the products.

________________ 6. It comes from the Greek words meaning “not to cut.”

________________ 7. He discovered the nucleus of the atom.

________________ 8. He discovered canal rays.

________________ 9. They later became known as protons.

_______________ 10. This is the negative end of a gas discharge tube.

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B. Explain how Rutherford came up with the nuclear model of the atom.

_________________________________________________________________

_________________________________________________________________

_________________________________________________________________

_________________________________________________________________

_________________________________________________________________

Compare your answers with those in the Answer Key on pages 28 and 29. Did you get aperfect score? If you did, that’s very good! If you didn’t, don’t worry. Just review the parts of thelesson that you didn’t understand very well. Afterward, you may move on to Lesson 2.

Let’s Remember

♦ The structure of matter had always puzzled human beings and theories on this structurewere proposed even by ancient Greek philosophers.

♦ The existence of indivisible units of matter was first proposed by the Greekphilosophers Leucippus and Democritus.

♦ John Dalton proposed the atomic theory which proved that matter is made up ofatoms. The postulates of the theory are:

1. Matter is made up of extremely small indestructible particles called atoms.

2. All the atoms of a given element are alike.

3. Atoms enter into combination with other atoms to form compounds but remainunchanged during ordinary chemical reactions.

4. Atoms can combine in simple numerical ratios.

♦ The discovery of the electron and the proton led scientists to conclude that the atom ismade up of even smaller particles.

♦ Sir Joseph John Thomson proposed the raisin-bread model of the atom, whichpresented the atom as a positive sphere in which electrons are embedded.

♦ Ernest Rutherford’s experiment on Thomson’s model proved that the atom has a smallcentral positive core moving around which are the electrons.

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LESSON 2

The Subatomic Particles

Isn’t it amazing how a tiny particle such as the atom could puzzle the greatest scientists ofour time? Do you have any idea how small the atom is? Each atom has an average diameter of0.1 to 0.5 nanometer (nm). The nanometer is 1/100000000 of a meter. If you get an atom with adiameter of 0.1 nm, you will need around 10 million of this atom in order to form a line across thisdot: •.

John Dalton described the atom as the ultimate particle into which matter can be divided. Ofcourse, his idea of the indivisibility of the atom no longer holds true because of experiments thatproved otherwise. The atom can be divided into much smaller particles called subatomicparticles.

In this lesson, you will find out the characteristics of the three subatomic particles—theelectron, proton and neutron. You will learn where in the atom these particles are located. Youwill also learn how to determine the number of electrons, protons and neutrons in an atom basedon its atomic number and mass number.

Let’s Try This

Look at the diagram below. Does it look familiar? It is Rutherford’s nuclear model of theatom. Can you identify which are the neutrons, the protons and the electrons in the diagram?

Find out if your answers are correct by reading the following section.

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Let’s Learn

What does the inside of an atom look like? After about 100 years of investigation into thecomposition of the atom, scientists now know that there are subatomic particles—these are theelectrons, neutrons and protons. These three particles are the fundamental units which make up allatoms.

Let us study these particles one by one.

Look at the following diagram. It shows the structure of a helium atom.

The center of the atom is made up of two types of particles—the neutron and the proton.The proton carries a positive charge and has a relative mass of 1.00734 or 1. The neutron iselectrically neutral, that is, it carries no charge. It has a relative mass of 1.00866 or 1. The protonand the neutron are referred to as nucleons because they are found in the nucleus of the atom.

Revolving around the nucleus are the electrons. The electron is much, much lighter than theneutron and proton. In fact, it is considered to have negligible mass. Its mass is only 0.00055amu. It carries a negative charge.

The following table shows the properties of each of the subatomic particles.

Electron e– –1 0.00055 9.10 × 10–28 g

Proton p +1 1.00734 1.673 × 10-24 g

Neutron n 0 1.00866 1.675 × 10-24 g

Subatomic Particle Electric ChargeSymbol Relative Mass Actual Mass

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Why is it that despite the presence of electrically charged particles—the negative electronsand the positive protons—an atom is electrically neutral?

Is your answer the same as this?

An atom is electrically neutral because the number of electrons in an atom is equal to thenumber of protons. The electric charges cancel each other out.

Let’s Review

Connect with a line the subatomic particle to its description.


Let’s Learn

How will you know the number of electrons, neutrons and protons in an atom? These canbe determined by looking at the atomic number and mass number of the atom. Take a look at thefollowing chemical symbol of an element.

Has a mass of 1.673 × 10–24 g

Carries no electric charge

Has a negative charge

Has a relative mass of 0.00055

Has a mass of 9.1110 × 10–28 g

Carries a positive charge



electron

proton

neutron

mass number

Z X A

symbol of the element atomic number

symbol of element

atomic number

mass number

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Since the mass of the atom is concentrated in the nucleus, the mass number A is equal tothe sum of the masses of the neutrons and protons in the atom. The atomic number Z on theother hand is the number of protons in the atom. Since the number of protons is equal to thenumber of electrons, the atomic number is also equal to the number of electrons.

Atomic number = number of protons = number of electrons

Mass number = number of protons + number of neutrons

From the atomic number and the mass number of an atom, you can get the number ofprotons, neutrons and electrons in the atom.

Let’s Try This

Try to complete the following table. The first item has been done for you.


Let’s Learn

The second postulate of the atomic theory states that atoms of the same element are alike.The existence of isotopes has contradicted this postulate.

Elements have been found to have atoms that differ in mass. Atoms of the same element thathave different masses are called isotopes of that element.

Na 11 23 11 11 12

H 1 0

Mg 24 12

O 16 8

Cl 17 18

Be 4 9

Element Atomic Number(Z)

Mass Number(A)

Proton Electron Neutron

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Hydrogen, for instance, has three isotopes—protium, deuterium and tritium. Where do youthink the difference among these atoms lies? Let’s look at their symbols:

Protium 1H1

Deuterium 1H2

Tritium 1H3

Can you now tell where the difference among these isotopes lies? The difference lies in thenumber of neutrons in each isotope. Protium has no neutron; deuterium has one neutron; tritiumhas two neutrons. Therefore, we can also say that isotopes of the same element differ in massnumber.

There are around 250 known naturally occurring isotopes. Around 1000 are made innuclear reactors. Isotopes have uses in medicine, especially in radiation therapy. Cobalt 60, forinstance, is used in cancer therapy while iodine 121 is used in goiter therapy.


A. Fill in the blanks.

1. The _______________of an atom indicates the number of protons in the atom.

2. The number of ___________ is equal to the number of ______________ in anatom.

3. A ____________ carries no electric charge.

4. An ____________ has negligible mass.

5.17

C135 and 17

C137 are __________ of chlorine.

6. The number of ___________ differentiates one isotope of an element fromanother.

7. A proton carries a ___________ charge.

8. The ____________ and _____________ carry most of the mass of the atom.

9. The ____________ takes up most of the space in an atom.

10. The symbol for the atomic number is _____.

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B. Complete the following table.

Compare your answers with those in the Answer Key on page 30. Did you get a perfectscore? If you did, that’s very good! If not, that’s okay. Just review the parts of the lesson that youdid not understand very well. Afterward, you may move on to Lesson 3.

Let’s Remember

♦ The three subatomic particles are the neutron, proton and electron. The neutron andproton are found in the nucleus of the atom; the electron moves around the atom.

♦ The electron carries a negative charge and has negligible mass.

♦ The proton has a positive charge and a relative mass of 1.

♦ The neutron carries no electric charge and has a relative mass of 1.

♦ An atom is electrically neutral because the number of protons is equal to the number ofelectrons.

♦ The atomic number is the number of protons in the atom while the mass number is thesum of the numbers of protons and neutrons in the atom.

♦ Isotopes are atoms of the same element with different masses.

6C12

6C13

8O18

8O17

7N14

16S32

20Ca40

19K39

47Ag108

82Pb207

Chemical Symbol Atomic Number Mass Number Proton Electron Neutron

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LESSON 3

Locating the Electron

In Lesson 2, you found out that the three subatomic particles are the neutron, proton andelectron. The proton and the neutron are more or less stable in the nucleus of the atom but theelectrons are constantly moving around the atom. The atoms, being of negligible mass, are fast-moving particles.

Do you think there is a way we could locate an electron inside an atom at any one time?Because it is fast-moving, an electron is very elusive. In fact, the exact location of an electron cannever be known. The most we could do is determine the probability or chance of an electronbeing in a particular spot in the atom.

In this lesson, you will find out how to do this. You will know about the different energylevels occupied by electrons in an atom. You will also be able to determine the number of orbitalsin a given energy level as well as the number of electrons in a sublevel.

Let’s Try This

Try to draw an atomic diagram based on the chemical symbol 8O16.

How will you do this? Based on the atomic number and mass number, you can see that thenumber of protons and electrons is 8 and the number of neutrons is 8.

In drawing the diagram of an atom, your main concern is the positions of the electrons. Eachelectron will occupy a specific level in the atom. Look at the diagram of the oxygen atom below:

Notice that there are two orbits around the nucleus of the atom. In the first (innermost) orbit,there are two electrons; in the second orbit, there are six electrons. Why were the electronsdistributed in this manner? You will find out in the following section.

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Let’s Learn

In 1913, a new model for the structure of the atom was proposed by Niels Bohr (1885–1962). According to Bohr, the electrons orbiting an atom could only exist at certain energy levels(or distances) from the nucleus of the atom. When an electron absorbs energy, it jumps to ahigher energy level (a greater distance from the nucleus) and as it returns to its original energylevel, it emits light.

This model of the atom explained several things, such as the periodic behavior of elements.According to this model, each orbit of the same size or energy (energy level or shell) could onlyhold so many electrons. For instance, the first shell or energy level can hold two electrons; thesecond energy level, eight electrons; the third energy level, eighteen electrons; and so on.

When one shell is filled, the electrons have to occupy higher energy levels. Chemicalproperties then are based on the number of electrons in the outermost shell. Elements with fulloutermost shells do not react; other elements take up or give up some electrons to achieve a fulloutermost shell.

Let’s study this in detail.

Look once again at the diagram of the oxygen atom on page 20. The orbit nearest thenucleus is the first energy level or shell. Notice that there are two electrons in this energy level.This energy level is filled up first and since it can hold only a maximum of two electrons, the restof the oxygen electrons have to go to the next energy level. The second energy level can hold amaximum of eight electrons. Since there are only six electrons to be distributed in this energylevel, this level is not yet full. Hence, the oxygen atom reacts with other atoms in such a way that itcan gain the two electrons it needs to fill its outermost energy level.

What elements are grouped along with oxygen in the periodic table? These are sulfur (16

S),selenium (

34Se), tellurium (

52Te) and polonium (

84Po). Look at the distribution of electrons in each

of these elements.

Did you notice that like oxygen, the four elements all have six electrons in the outermostenergy level? This similarity accounts for the similarity in their chemical properties.

Sulfur Selenium

Tellurium Polonium

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Have you ever been to a basketball game? Notice that when you go to a basketball game ina stadium, there are several seating sections. There’s the ringside, which is nearest the basketballcourt. There are also the lower box, upper box, etc.

Look at the diagram below which shows the seating sections in a stadium:

Notice that each section is composed of several seats. However, the nearer the section is tothe basketball court, the fewer the seats in that section. This is the same thing with the energylevels in an atom. The nearer an energy level is to the nucleus of the atom, the fewer the electronsit can accommodate.

Let’s Learn

How will you know the maximum number of electrons an energy level can hold? Theformula for this is 2n2, where n is the number of the energy level. Look at the diagram below:

Gallery

Upper box

Lower box

Ringside

Hardcourt

n = 1 n = 2 n = 3 n = 4

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Keep in mind that the higher the number of the energy level, the farther it is from the nucleusof the atom and the more electrons it can hold. The following is a table of the number of electronsthe energy levels of an atom can hold:

Let’s Review

State the maximum number of electrons the following energy levels can hold:

Energy Level Maximum Number of Electrons

4 ______

6 ______

9 ______


Let’s Learn

As I mentioned in the beginning of this lesson, there is no known method for knowing theexact location of an electron in an atom. But we can determine the probability of finding an atomin a region in space. This region in space where an electron is most likely to be found is calledatomic orbital.

An atomic orbital may be any of four different shapes. These are:

♦♦♦♦♦ Sharp (s)—spherical; there can only be one of this in an energy level.

♦♦♦♦♦ Principal (p)—dumbbell-shaped; a maximum of three can be found in an energy level.

♦♦♦♦♦ Diffuse (d)—four lobe-shaped; a maximum of five can occupy an energy level.

♦♦♦♦♦ Fundamental (f)—six lobe-shaped; a maximum of seven can be found in an energylevel.

Scientists have found out that only two electrons can share the same orbital, each spinning ina direction opposite to that of the other electron. This is known as Pauli’s exclusion principle,which, stated in a different way, says that only two electrons can occupy the same atomic orbitalat the same time.

1 2

2 8

3 18

4 32

5 50

Maximum Number of ElectronsEnergy Level (n)

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Thus, based on Pauli’s exclusion principle, we can determine the number of electrons in anorbital. Look at the following table:

Do you think that the number of energy level is related to the number of atomic orbitals? Ofcourse! Look at the following table that shows the relationship between the two:


Given the following elements, identify (a) the number of energy levels the electrons of eachelement will occupy and (b) the number of electrons in the outermost energy level of eachelement.

1. 1H

2. 5B

3. 9F

4. 17

Cl

5. 12

Mg

Compare your answers with those in the Answer Key on page 31. Did you get a perfectscore? If you did, that’s very good! If not, don’t worry. Just review the parts of the lesson thatyou did not understand very well before you move on to the next part of the module.

s 1 1(2) = 2

p 3 3(2) = 6

d 5 5(2) = 10

f 7 7(2) = 14

Number of ElectronsOrbital Number of Atomic Orbitals

Energy LevelAtomic Orbitals

Present

Number ofElectrons inthe Orbitals

1

2

3

4

Total Numberof Electrons

s

s

p

s

p

d

s

p

d

f

2

2

6

2

6

10

2

6

10

14

2

8

18

32

25

Let’s Remember

♦ Niels Bohr’s model of the atom states that the electrons orbiting an atom could onlyexist at certain energy levels from the nucleus of the atom.

♦ Each energy level of the atom can hold only a specific number of electrons, accordingto the formula 2n2 where n is the number of the energy level.

♦ The higher the number of the energy level, the farther it is from the nucleus and thehigher the number of electrons it can hold.

♦ The region in space where an electron is most likely to be found is called an atomicorbital. An atomic orbital may be any of four shapes—sharp (s), principal (p), diffuse(d) and fundamental (f).

Well, this is the end of the module. Congratulations for finishing it! Did you enjoy studying it?Did you learn a lot from it? The following is a summary of its main points to help you rememberthem better.

Let’s Sum Up

This module tells us that:

♦ Matter is made up of atoms, the smallest units that retain the properties of matter.

♦ The atom has three subatomic particles—the neutron, proton and electron.

♦ An atom is electrically neutral because the number of protons is equal to the number ofelectrons in the atom.

♦ The atomic number of an atom is equal to the number of protons in the atom and isalso equal to the number of electrons in that atom.

♦ The mass number of an atom is the sum of the masses of the protons and neutrons inthe atom.

♦ Electrons that orbit an atom occupy certain energy levels or distances from the nucleusof the atom. The higher the energy level, the farther it is from the nucleus and the moreelectrons it can hold.

♦ The maximum number of electrons an energy level can hold is equal to 2n2, where n isequal to the number of the energy level.

♦ The region in space where an electron is most likely to be found is called atomicorbital.

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What Have You Learned?

A. Identify what or who is being described in each of the following.

________________ 1. Spherical orbital

________________ 2. States that only two electrons can occupy an atomicorbital at the same time

________________ 3. Proposed the atomic theory

________________ 4. A subatomic particle that does not carry any electriccharge

________________ 5. The region in space where an electron is most likely to befound

________________ 6. Dumbbell-shaped orbital

________________ 7. Also equal to the number of electrons in an atom

________________ 8. Atoms of the same element with different masses

________________ 9. Has negligible mass

________________ 10. Energy level that can carry a maximum of eighteenelectrons

B. Write the letter of the correct answer on the line before the number.

____ 1. He proposed that matter was composed of four elements—earth, water, airand fire.

a. Democritusb. Empedoclesc. Rutherfordd. Dalton

____ 2. Atoms are composed of the subatomic particles _____ .

a. electrons and protonsb. protons and neutronsc. electrons, protons and neutronsd. protons

____ 3. The region around the nucleus is composed of _____ .

a. protons and electronsb. protons and neutronsc. electronsd. protons

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____ 4. The mass number less the atomic number gives the number of _____ .

a. electronsb. electrons and protonsc. neutronsd. neutrons and protons

____ 5. The mass numbers of isotopes are _____ .

a. differentb. equalc. divisible by each otherd. in small whole number ratios

____ 6. Which of the following statements correctly describes an electron?

a. It is positively charged.b. It is found in the nucleus.c. It has a mass equal to that of the neutron.d. It is negatively charged.

____ 7. If an element is described by these data: Z = 19 and A = 40, which of thefollowing statements is true of the element?

a. It has 19 protons and 20 neutrons.b. It has 19 protons and 19 neutrons.c. It has 21 electrons and 19 protons.d. It has 19 electrons and 21 neutrons.

____ 8. An energy level can hold _____ s orbital(s).

a. oneb. twoc. threed. four

____ 9. An atom has an atomic number of 17 and a mass number of 35.Which ofthe following statements is correct?

a. The atom has 17 electrons and 17 neutrons.b. The atom has 17 electrons and 18 neutrons.c. The atom has 17 electrons and 18 protons.d. The atom has 18 electrons and 17 neutrons.

____10. For the isotopes 20

Ca40 and 20

Ca42, which statement is not true?

a. There are 20 electrons in both isotopes.b. There are 20 neutrons in both isotopes.c. The number of protons in both isotopes is 20.d. Ca40 has 20 neutrons and Ca42 has 22 neutrons.

Compare your answers with those found in the Answer Key on page 32. If you got aperfect score, that’s very good. This means that you learned a lot from this module. If you didnot, just review the lessons you did not understand very well.

28

Answer Key

A. Let’s See What You Already Know (pages 2–3)

1. d One of the assumptions of Dalton’s atomic theory is that atoms of the sameelement are alike.

2. c

3. b

4. b The atomic number indicates the number of electrons present in the atom.Since the atom is electrically neutral, the number of electrons is also equal tothe number of protons.

5. d The mass number gives the sum of the protons and neutrons in the atom.

6. d

7. b

8. a

9. c

10. b Isotopes differ in the number of neutrons.

B. Lesson 1

Let’s Review (page 9)

Cathode rays are negatively charged particles while canal rays are positivelycharged particles. The existence of these two particles violated the first postulate of theatomic theory because it showed that the atom is not indivisible—it is made up of evensmaller particles.

Let’s See What You Have Learned (pages 12–13)

A. 1. John Dalton

2. cathode rays

3. Sir Joseph John Thomson

4. Empedocles

5. law of conservation of mass

6. atom

7. Ernest Rutherford

8. Eugen Goldstein

9. canal rays

10. cathode

29

B. Rutherford was able to come up with the nuclear model of the atom based on theresults of the experiment he conducted to test Thomson’s raisin-bread model ofthe atom. He allowed a beam of alpha particles to pass through a gold foil. Someof the alpha particles went undeflected to the cathode end of the tube; othersbounced back with large angles of deflection; still others went to the cathode endwith only slight deflections. This made him conclude that the atom is made up ofmostly empty space with a positive core or nucleus and negative electrons movingaround the nucleus. The undeflected alpha particles passed through the emptyspace; those which bounced back hit the positive nucleus; while those which hadslight deflections hit the very light electrons.

C. Lesson 2


Let’s Try This (page 17)

Has a mass of 1.673 × 10–24 g

Carries no electric charge

Has a negative charge


Has a mass of 9.1110 × 10–28 g

Carries a positive charge



electron

proton

neutron

Element Atomic Number(Z)

Mass Number(A)

Proton Electron Neutron

Na 11 23 11 11 12

H 1 1 1 1 0

Mg 24 12 12 12 12

O 8 16 8 8 8

Cl 17 35 17 17 18

Be 4 9 4 4 5

30

Let’s See What You Have Learned (pages 18–19)

A. 1. atomic number

2. protons; electrons (Answers may be in any order.)

3. neutron

4. electron

5. isotopes

6. neutrons

7. positive

8. protons; neutrons (Answers may be in any order.)

9. electron (This is because it is moving around the atom all the time.)

10. Z

B.

6C12 6 12 6 6 6

6C13 6 13 6 6 7

8O18 8 18 8 8 10

8O17 8 17 8 8 9

7N14 7 14 7 7 7

16S32 16 32 16 16

16

20Ca40 20 40 20 20 20

19K39 19 39 19 19 20

47Ag108 47 108 47 47 61

82Pb207 82 207 82 82 125

Chemical Symbol Atomic Number Mass Number Proton Electron Neutron

31

D. Lesson 3


Energy Level Maximum Number of Electrons

4 32

6 72

9 162

Let’s See What You Have Learned (page 24)

1. a. number of electrons = 1; therefore, only 1 energy level is occupied.

b. Since there is only 1 electron, 1 electron will occupy the outermost energylevel.

2. a. number of electrons = 5; two electrons will occupy the first energy level,three will occupy the second energy level. Therefore, 2 energy levels will beoccupied by the electrons.

b. There are 3 electrons in the outermost energy level.

3. a. number of electrons = 9; two will occupy the first energy level, seven willoccupy the second energy level. Therefore, 2 energy levels will be occupiedby the electrons.


4. a. number of electrons = 17; two will occupy the first energy level, eight willoccupy the second energy level and seven will occupy the third energy level.Therefore, 3 energy levels will be occupied by the electrons.


5. a. number of electrons = 12; two will occupy the first energy level, eight willoccupy the second energy level, two will occupy the third energy level.Therefore, 3 energy levels will be occupied by the electrons.


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E. What Have You Learned? (pages 26–27)

A. 1. s orbital

2. Pauli’s exclusion principle

3. John Dalton

4. neutron

5. atomic orbital

6. d orbital

7. atomic number

8. isotopes

9. electron

10. third energy level

B. 1. b

2. c The subatomic particles found in the atom are the electrons, protonsand neutrons.

3. c The region around the nucleus is where electrons may be found.

4. c The mass number gives the number of protons and neutrons while theatomic number indicates the number of protons/electrons. Therefore,the mass number less the atomic number gives the number of neutrons.

5. a Isotopes have the same atomic number but different mass numbers.

6. d

7. d

8. a An energy level can hold a maximum of one s orbital.

9. b The atomic number indicates the number of protons which is also equalto the number of electrons. The mass number minus the atomic numbergives the number of neutrons.

10. b Isotopes differ in the number of neutrons.

33

Glossary

Alpha ray A positively charged particle identical with the helium atom, having twoprotons and two neutrons.

Anode The positive end or electrode.

Atom The smallest particle of an element that has the same properties as the element.

Cathode The negative end or electrode.

Chemical reaction A process that causes a change in one or more substances, resultingin the formation of one or more new substances.

Compound Two or more elements combined in fixed proportions.

Deflection A turning aside or off course.

Indestructible Cannot be destroyed or broken down into smaller parts.

Model A description or analogy used to help visualize something that cannot be directlyobserved.

Negligible Extremely small or unimportant.

Nuclear Pertaining to the nucleus of the atom.

Nucleus The center of an atom.

Postulate A statement accepted as true as the basis for an argument or reasoning.

Probability The chance that a given event will occur.

Product The substance produced in a chemical reaction.

Proportion The relation of one part to another or the whole in terms of quantity.

Radiation therapy Treatment of a disease by exposing the patient to particles emitted bya radioactive substance.

Radioactive Capable of emitting high-energy particles through disintegration of thenucleus of the atom.

Ratio The relationship in quantity between two or more things; similar to proportion.

Reactant The starting material in a chemical reaction.

Relative mass Average mass.

Subatomic Of, relating to or being particles smaller than the atom.

Vacuum Devoid of matter; empty space.

34

References

Keenan, Charles W., Jesse H. Wood and Donald C. Kleinfelter. General CollegeChemistry. 5th ed. New York: Harper and Row Publishers, 1976.

Mortimer, Charles E. Chemistry: A Conceptual Approach. New York: Van NosstrandCo., 1975.

Redmore, Fred. Fundamentals of Chemistry. New Jersey: Prentice-Hall, Inc., 1979.

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Atom a Closer Look