Unit 5: Atomic Structure and the Periodic Table

of Elements

5.1 Atoms are the smallest form of element s

What we should already know:› All matter is made of atoms› Elements are the simplest substances

o What we will learn:o Where atoms are found and how they are

named o About the structure of atomso How ions are formed from atoms

Greek philosophers use to believe that everything on Earth was made up of air, water, fire, and earth or a combination of those substances.

Today’s chemists know here are about 100 basic substances, or elements, which make up everything we see and touch.› Some of these elements appear by

themselves› Some appear in combination with others

Types of Atoms in Earth’s Crust and Living Things:

Hydrogen (H) makes up about 90% of the mass of the universe

Hydrogen (H) makes up only about 1% of the earth’s crust

Look at the graphs on page 138:› Oxygen (O) makes up the majority of both

humans and the earth’s crust› After oxygen (O) there are many differences in

our make-up All living things are composed primarily of:

oxygen (O), carbon (C), hydrogen (H) , and nitrogen (N)

Names and Symbols of Elements:

Each element has its own unique atomic symbol:› Some elements use the first letter of the

name: Oxygen = O Hydrogen = H Sulfur = S› Other elements use the first letter plus

another letter found in the name: Aluminum = Al Platinum = Pt Zinc = Zn

› Some elements use the letters from the Latin word such as: gold = Au Lead = Pb

No matter the letters used, the first letter is always UPPER CASE and the second letter is always lower case

Each element is made of a different atom

John Dalton proposed that each element is made of tiny particles called atoms.

All of the atoms of a particular element are identical but are different from atoms of all other elements.

The Structure of an Atom Atoms contain charged particles which

can be (+) or (-)› Particles with like charges repel each other

(push away from)› Particles with different charges attract

each other (drawn to) Atoms are composed of three types of

particles:› Protons› Neutrons› Electrons

Protons:› Carry a (+) charge› Are found in the nucleus of the atom› Make the element what it is

If you change the number of protons, you change the element

Neutrons:› Carry a neutral charge (no charge)› Are found in the nucleus of the atom› Have about the same mass as a proton

Electrons:› Carry a (-) charge› Are found outside the nucleus of the atom› Determine how elements will bond with

each other

Atomic ModelElectrons (-)

Protons (+)

Neutrons

Size and attraction: About 10-10 meter in diameter

› You could fit millions of atoms in the size of the period in your book

Electrons are much smaller than protons and neutrons› About 2000 times smaller

Electrons move about the nucleus very quickly› Impossible to pin point the location at any

time› Stay attracted to the nucleus because of

the (+) protons

Atomic Number:

The atomic number of an atom indicates the number of protons in the nucleus.

You can not change the atomic number of an element and still have that same element

The atomic number of hydrogen (H) will always be 1.

The atomic number of gold (Au) will always be 79.

Atomic mass number

The total number of protons and neutrons in an atom’s nucleus.

Not all atoms have the same number of neutrons:› Chlorine (Cl) has 17 protons but some atoms

have 18 neutrons, others may have 20. When atoms of the same element have a

different number of neutrons they are called isotopes.› To find the number of neutrons you must

subtract the atomic number from the atomic mass number.

Ions: A neutral atom has an equal number of

protons (+) and electrons (-).› The negative charges and the positive

charges balance each other. An ion is formed when an atom loses

or gains one or more electrons.› By gaining or losing electrons, the balance

of (+) and (-) is no longer there.

Positive Ions: When an atom loses one or more

electrons, it takes on a positive charge.› Example: page 142› A neutral atom of sodium contains 11

protons and 11 electrons—the charges balance each other.

11 (+)11 (-)Neutral charge

When the sodium atom loses an electron, there are more (+) charges than (-) giving the ion a + charge

To indicate or write a positive charged atom, you add a + sign to the right side of the atomic symbol.

Na+

If an atom loses more than one electron, you add the number lost to the atomic symbol before the + sign

Na 2+

Negative Ions: Negative ions are formed opposite to

the positive ions—Electrons are added to the atom giving it a negative charge.

When indicating a negative electron, you add a – sign to the top right of the atomic symbol.

Cl- If more than one electron is added, you

write the number with the sign.

Cl2-

5.2: Elements make up the periodic table

Not all atoms of an element have the same atomic mass number (total number of protons and neutrons)

The atomic mass of the atom is the average mass of all the element’s isotopes (same atom-different number of neutrons.)

Mendeleev’s Periodic Table page 146

Early 1800’s, Dmitri Mendeleev wanted to organize the elements based on their physical and chemical properties.

Created the first periodic table by arranging the elements by:› Similarities made up each row› Increase down each column by atomic

mass

Modern periodic table

Page 148-149 in your book Elements are arranged by atomic

number (number of protons) Elements with Similar properties are

arranged in columns

Reading the periodic table

Each square represents an individual element:

Atomic number (number of protons)

Atomic symbol

Name Atomic mass

The color of the letters indicates the physical state of the element at room temperature:› White = gas› Blue = liquid› Black = solid

Your table is also divided by the colors:› Green = Nonmetal› Yellow = Metal› Purple = Metalloid

Groups Each column on the table is known as a

group or family because they appear to be related.

The halogens-group 17 tend to combine well with groups one and two.

Similar chemical properties Different physical properties

GasesLiquidSolids

Periods Each horizontal row is called a period. Properties of the elements change in a

“predictable” way from one end to the other.

Elements on the far left are metals. Elements on the far right are

nonmetals.

What else is on the table?An atom’s size is important because it affects how the atom will react with another atom. What is the pattern in the table for atomic size?

Periodic table densities:

Densities of the elements follows a pattern with the least dense on the left and right and the most dense in the middle.

5.3: The periodic table is a map of the elements

Just like a map of the United States, the periodic table is a map showing:› Metals (left)› Metalloids (middle)› Nonmetals (right)

Nonmetals does not count hydrogen (H) Your book shows these regions in:

› Yellow (metals)› Purple (metalloids)› Green (nonmetals)

Reactive:› Indicates how likely an element is to

undergo a chemical change.› Most elements are somewhat reactive and

combine with other materials› Groups 1 and 17 are the most reactive› Group 18 is the least reactive

Metals: Elements that conduct electricity and

heat well Have shiny appearance Can be shaped easily by pounding,

bending, or being drawn into a long wire

Solids at room temperature except for mercury

Reactive and transition

Reactive metals

Groups 1 and 2 Group 1 is alkali metals

› Very reactive› When exposed to air-rapid reactions occur› Sodium and potassium are stored in oil

Group 2 is alkaline earth metals› Less reactive than alkali metals› More reactive than most other metals› Calcium and magnesium

Transition metals Groups 3-12 Generally less reactive than most other

metals Properties make them useful in

industry and technology Commonly combined to form alloys

such as brass (combination of copper and zinc)

Includes common metals such as copper, gold, silver, and iron

Rare Earth Elements

Top row of the two rows of metals that are usually separated from the rest of the table

AKA: lanthanides Thought to be rare in amount but have

been discovered as more common just hard to isolate in pure form.

Nonmetals Found on the right side Properties are generally the opposite of

the metals and vary more:› Many are gases at room temperature› Some are solids› One is a liquid

Solids are dull and cannot be shaped by hammering or drawn into wires

Generally poor conductors of heat and electricity

Halogens:› Greek meaning “forming salts”› Group 17› Very reactive nonmetals that form

compounds called salts with many metals› Often used to kill harmful microorganisms

Noble Gases:› AKA: inert gases› Group 18› Almost never react with other elements› Examples: neon, krypton, xenon, argon

Metalloids Elements that have properties of both

metals and nonmetals Often make up the semiconductors of

electrical devices› Semiconductors conduct electricity under

some circumstances but not others

Some atoms change their identity

Radioactivity:› Process by which atoms produce energy

and particles› Isotopes (atoms with different numbers of

neutrons) are radioactive if the nucleus has too many or too few neutrons

› When this happens, particles are produced from the nucleus of the atom to restore the balance.

Radioactive Decay Radioactive decay:

› When radioactive atoms produce energy and particles from their nuclei, the identity of the atom changes because the number of protons changes.

› Steady rate of change Half life:

› Amount of time it takes for one-half of the atoms in a particular sample to decay.

› Not affected by temperature or pressure› Specific to each isotope

Half life example:

1000 atoms of kryptonite› Half life of kryptonite is 1 year

After 1 year, you would have ½ or 500 atoms of kryptonite.

After 2 years, you would have ½ of the atoms from year 1 or 250 atoms of kryptonite.

After 3 years, you would have ½ of the atoms from year 2 or 125 atoms of kryptonite.

Scientific notation “short hand” way for scientists to write

extremely large or small numbers. It will always be a number between 1

and 10 and a power of 10. To write 6,400,000,000 in scientific

notation you move your decimal to the left until you end up with 6.4.› Count how many spaces you moved (9)› Re-write the number as 6.4 x 109

To write 0.0000023 in scientific notation:› Count how many spaces you move the

decimal to the right to get to 2.3› 6 spaces› Re-write the number as 2.3 x 10-6

When going to scientific notation:› If you move your decimal to the left, your

exponent is positive› If you move your decimal to the right, your

exponent is negative

When going from scientific notation:› If you exponent is positive you move your

decimal to the right› If your exponent is negative you move

your decimal to the left.