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Chapter 17: Properties of Atoms and the

Periodic Table

Unit 4: The Nature of Matter

Table of ContentsTable of Contents

17.3: The Periodic Table

17.1: Structure of the Atom

17.2: Masses of Atoms

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Scientists havedeveloped theirown shorthand for

dealing with long,complicated names.

Chemical symbolsconsist of one capital

letter or a capitalletter plus one or twosmaller letters.

ScientificShorthand17.117.1

Structure of the AtomStructure of the Atom

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For some elements, the symbol is the firstletter of the element's name.

For other elements, the symbol is the firstletter of the name plus another letter from its

name.

ScientificShorthand17.117.1


B

ecause scientists worldwide use this system,everyone understands what the symbolsmean.

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An element is matter that is composed of onetype ofatom, which is the smallest piece ofmatter that still retains the property of the

element.

Atoms are composed ofparticles called protons,

neutrons, and electrons.

Atomic Components17.117.1


Click image to view movie

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Protons and neutrons are found in a smallpositively charged center of the atom calledthe nucleus that is surrounded by a cloudcontaining electrons.

Protons areparticleswith anelectricalcharge of1+.



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Electrons are particles with an electricalcharge of 1.



Neutrons are neutral particles that do nothave an electrical charge.

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Protons and neutrons are made up of smallerparticles called quarks.

QuarksEven Smaller Particles17.117.1


So far, scientists have confirmed theexistence of six uniquely different quarks.

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To study quarks, scientists accelerate chargeparticles to tremendous speeds and then forcethem to collide withor smash into

protons. This collision causes the proton tobreak apart.

Finding Quarks17.117.1


The particles that result from the collision

can be detected by various collection devises.

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Scientists found five quarks and hypothesizedthat a sixth quark existed. However, it took ateam of nearly 450 scientists from around the

world several years to find the sixth quark.

The Sixth Quark17.117.1


The tracks of the sixth quark were hard todetect because only about one billionth of a

percent of the proton collisions performedshows a presence of a sixth quark.

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Scientists and engineers use models torepresent things that are difficult tovisualizeor picture in your mind.

ModelsTools for Scientists17.117.1


Scaled-down models allow you to see eithersomething too large to see all at once, orsomething that has not been built yet.

Scaled-up models are often used to visualizethings that are too small to see.

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To study the atom, scientists have developedscaled-up models that they can use tovisualize how the atom is constructed.

ModelsTools for Scientists17.117.1


For the model to be useful, it must support allof the information that is known about matterand the behavior of atoms.

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In the 1800s, John Dalton, an Englishscientist, was able to offer proof that atomsexist.

The Changing Atomic Model17.117.1


Another famous Greek philosopher, Aristotle,disputed Democritus's theory and proposedthat matter was uniform throughout and wasnot composed of smaller particles.

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In the 1800s, John Dalton, an Englishscientist, was able to offer proof that atomsexist.

The Changing Atomic Model17.117.1


Dalton's model of theatom, a solid spherewas an early model ofthe atom.

The model haschanged somewhatover time.

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By 1926, scientists had developed theelectron cloud model of the atom that isin use today.

The Electron Cloud Model17.117.1


An electroncloud is the areaaround thenucleus of an

atom where itselectrons are mostlikely found.

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The electron cloud is 100,000 times largerthan the diameter of the nucleus.

The Electron Cloud Model17.117.1


In contrast, each electron in the cloud ismuch smaller than a single proton.

Because an electron's mass is small and theelectron is moving so quickly around the

nucleus, it is impossible to describe its exactlocation in an atom.

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Section CheckSection Check

17.117.1

Question 1

A. atom

B. quark

C. neutron

D. proton

Which is the smallest piece of matter that

still retains the property of the element?

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Answer

17.117.1


The answer is A. An atom is the smallest

piece of matter that still retains the property

of the element.

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17.117.1

Question 2

A. protons and electrons

B. protons and neutrons

C. neutrons and electrons

D. quarks and electrons

What particles are found in the nucleus of an

atom?

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Answer

17.117.1


The answer is B.

Electrons are

located in an

electron cloud

surrounding the

nucleus of theatom.

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17.117.1

Question 3

What is the name of the small particles that

make up protons and neutrons?

Answer

Protons and neutrons are made of smaller

particles called quarks.

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Atomic Mass

Masses of AtomsMasses of Atoms

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Atomic Mass

The nucleus containsmost of the mass ofthe atom becauseprotons and neutronsare far more massivethan electrons.

The mass of a proton

is about the same asthat of a neutronapproximately


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Atomic Mass

The mass of each isapproximately1,836 times greater

than the mass of theelectron.


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Atomic Mass

The unit of measurement used for atomicparticles is the atomic mass unit (amu).

The mass of a proton or a neutron is almostequal to 1 amu.

The atomic mass unit is defined as one-

twelfth the mass of a carbon atom containingsix protons and six neutrons.


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Protons Identify the Element

The number of protons tells you what type ofatom you have and vice versa. For example,every carbon atom has six protons. Also, allatoms with six protons are carbon atoms.

The number of protons in an atom is equal to

a number called the atomic number.


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Mass Number

The mass number of an atom is the sum ofthe number of protons and the number ofneutrons in the nucleus of an atom.


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Mass Number

If you know the mass number and the atomicnumber of an atom, you can calculate thenumber of neutrons.


number of neutrons = mass number atomic number

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Isotopes

Not all the atoms of an element have thesame number of neutrons.

Atoms of the same element that havedifferent numbers of neutrons are calledisotopes.


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Identifying Isotopes

Models of two isotopes of boron are shown.Because the numbers of neutrons in theisotopes are different, the mass numbers arealso different.

You use the name ofthe element followedby the mass number of

the isotope to identifyeach isotope: boron-10 and boron-11.


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Identifying Isotopes

The average atomic mass of an element isthe weighted-average mass of the mixture ofits isotopes.

For example, four out of five atoms of boronare boron-11, and one out of five is boron-10.


To find the weighted-average or the averageatomic mass of boron, you would solve the

following equation:

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17.217.2

Question 1

How is the atomic number of an element

determined?

Answer

The atomic number of an element is equal to

the number of protons in an atom of that

element.

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17.217.2

Question 2

The element helium has a mass number of 4

and atomic number of 2. How many

neutrons are in the nucleus of a helium

atom?

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Answer

17.217.2


Recall that the atomic number is equal to the

number of protons in the nucleus. Since the

mass number is 4 and the atomic number is

2, there must be 2 neutrons in the nucleus of

a helium atom.

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17.217.2

Question 3

How much of the mass of an atom is

contained in an electron?

Answer

The electrons mass is so small that it is

considered negligible when finding the

mass of an atom.

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Organizing the Elements

Periodic means "repeated in a pattern."

In the late 1800s, Dmitri Mendeleev, a

Russian chemist, searched for a way toorganize the elements.

The Periodic TableThe Periodic Table

When he arranged all the elements known

at that time in order of increasing atomicmasses, he discovered a pattern.

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Organizing the Elements

Because the pattern repeated, it wasconsidered to be periodic. Today, thisarrangement is called a periodic table of

elements.

In the periodic table, the elements are

arranged by increasing atomic number and bychanges in physical and chemical properties.


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Mendeleev's Predictions

Mendeleev had to leave blank spaces in hisperiodic table to keep the elements properlylined up according to their chemical

properties.

He looked at the properties and atomicmasses of the elements surrounding theseblank spaces.


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From thisinformation, hewas able topredict the

properties andthe massnumbers of newelements thathad not yet beendiscovered.


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This table showsMendeleev'spredictedproperties forgermanium,which he calledekasilicon. His

predictionsproved to beaccurate.


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Improving the Periodic Table

On Mendeleev's table, the atomic massgradually increased from left to right. If you

look at the modern periodic table, you willsee several examples, such as cobalt andnickel, where the mass decreases from left toright.


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Improving the Periodic Table

In 1913, the work of Henry G.J. Moseley, ayoung English scientist, led to thearrangement of elements based on their

increasing atomic numbers instead of anarrangement based on atomic masses.


The current periodic table uses Moseley'sarrangement of the elements.

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The Atom and the Periodic Table

The vertical columns in the periodic table arecalled groups, or families, and are numbered

1 through 18.


Elements in each group have similarproperties.

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Electron Cloud Structure

In a neutral atom, the number of electrons isequal to the number of protons.


Therefore, a carbon atom, with an atomicnumber of six, has six protons and six

electrons.

17.317.3

Th P i di T blTh P i di T bl

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Scientists have found that electrons withinthe electron cloud have different amounts ofenergy.


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Scientists model the energy differences of theelectrons by placing the electrons in energylevels.

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Energy levels nearer the nucleus have lowerenergy than those levels that are farther away.


Electrons fill these energy levels from theinner levels (closer to the nucleus) to the

outer levels (farther from the nucleus).

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Elements that are in the same group have thesame number of electrons in their outerenergy level.


It is the number of electrons in the outerenergy level that determines the chemical

properties of the element.

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Energy Levels

The maximum number of electrons that canbe contained in each of the first four levels isshown.


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Energy Levels


For example, energy level one can contain amaximum of two electrons.

A complete and stable outer energy level willcontain eight electrons.

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Rows on the Table

Remember that the atomic number found onthe periodic table is equal to the number ofelectrons in an atom.


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Rows on the Table


The first row has hydrogen with one electronand helium with two electrons both in energylevel one.

Energy level one can hold only two electrons.Therefore, helium has a full or completeouter energy level.

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Rows on the Table

The second row begins with lithium, whichhas three electronstwo in energy level oneand one in energy level two.


Lithium is followed by beryllium with two

outer electrons, boron with three, and so onuntil you reach neon with eight outer electrons.

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Electron Dot Diagrams

Elements that are in the same group have thesame number of electrons in their outerenergy level.


These outer electrons are so important indetermining the chemical properties of anelement that a special way to represent them

has been developed.

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Electron Dot Diagrams

An electron dot diagram usesthe symbol of the element anddots to represent the electrons inthe outer energy level.


Electron dot diagrams are usedalso to show how the electrons inthe outer energy level are bonded

when elements combine to formcompounds.

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Same GroupSimilar Properties

The elements inGroup 17, thehalogens, haveelectron dot

diagrams similarto chlorine.


All halogens have

seven electrons intheir outer energylevels.

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A common property of the halogens is theability to form compounds readily withelements in Group 1.


The Group 1 element, sodium, reacts easily

with the Group 17 element, chlorine. The result is the

compound sodium

chloride, orNaClordinarytable salt.

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Not all elements will combine readily withother elements.


The elements inGroup 18 havecomplete outerenergy levels.

This special

configuration makesGroup 18 elementsrelatively unreactive.

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Regions on the Periodic Table

All of the elements in the blue squares aremetals.


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Those elements on the right side of the periodictable, in yellow, are classified as nonmetals.

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The elements in green are metalloids orsemimetals.

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Elements in the Universe

Using the technology that is available today,scientists are finding the same elementsthroughout the universe.


Many scientists believe that hydrogen andhelium are the building blocks of other

elements.

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Elements in the Universe

Exploding stars, or supernovas, givescientists evidence to support this theory.


Many scientists believe that supernovas havespread the elements that are found throughout

the universe.

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17.317.3

Question 1

How are the elements arranged in the

periodic table?


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Sec o C ecSec o C ec

17.317.3

Answer

The elements are arranged by increasing

atomic number and by changes in physical

and chemical properties.


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17.317.3

Question 2

What do the elements in a vertical column of

the periodic table have in common?


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Answer

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The vertical columns in the periodic table

are called groups; elements in the same

group have similar properties, such aselectrical conductivity.

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17.317.3

Answer

The dots represent the electrons in the outer

energy level.

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End of Chapter Summary File

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17 Chapter