Chapter 18—PROPERTIES OF ATOMS

AND THE PERIODIC TABLE

Section—1: STRUCTURE OF THE ATOM

ATOMIC COMPONETS

An element is matter that is composed of 1 type of atom.

An atom is the smallest piece of matter that still retains the property of the element.

EXAMPLE—the element silver = ONLY silver atoms; the element hydrogen = ONLY hydrogen atoms

ATOMIC MODELSDEMOCRITUS—Greek philosopher—400 B.C.

THOMSON—English physicist—1904 (Pos-orange- p, Neg-red-e)

ATOMIC MODELSRUTHERFORD—British physicist—1911 (Majority of atom’s mass in nucleus)

BOHR—Danish physicist—1913 (Electrons in fixed orbits; nucleus had

+p, 0 n

•PROTONS + NEUTRONS = NUCLEUS (+ Charge)•ELECTRONS = OUTSIDE THE NUCLEUS (- Charge)

• Protons=Positive• Neutrons=Neutral (NO CHARGE)• Electrons=Negative

Chapter—18—PROPERTIES OF ATOMS

AND THE PERIODIC TABLE

Section 2—MASSES OF ATOMS

ATOMIC MASS

• The NUCLEUS contains most of the mass of an atom, because PROTONS AND NEUTRONS are far more massive than electrons.

• Protons and neutrons are about 2,000 times more massive than an electron.

• The unit of measurement used for atomic particles is the atomic mass unit or amu.

ATOMIC MASS UNIT (AMU)The mass of a proton or a neutron is equal to 1

amu.

Example---Atomic Mass Unit for CARBONC = 12, BECAUSE

6 PROTONS + 6 NEUTRONS = 12 amu’s

PROTONS IDENTIFY THE ELEMENTThe number of protons tell you what type of atom

you have.Ex. All atoms with 6 protons = Carbon; All atoms

with 8 protons = Oxygen The number of protons = the atomic numberCarbon = 6 protons = atomic number is 6Oxygen = 8 protons = atomic number is 8

MASS NUMBER OR ATOMIC MASS

The mass number of an atom is the sum of the number of PROTONS and the number of NEUTRONS in the nucleus of an atom.

MASS NUMBER (ATOMIC MASS) = Protons + Neutrons

PERIODIC TABLE

PERIODIC TABLE

NEUTRONSIf you know the mass number (atomic mass) and

the atomic number of an atom, you can calculate the number of NEUTRONS.

NUMBER OF NEUTRONS = mass number (atomic mass)- atomic number

ORNUMBER OF NEUTRONS = BIG - LITTLE

ISOTOPESNot all atoms of an element have the same

number of NEUTRONS.

Atoms of the same element that have a different number of neutrons are called ISOTOPES. (Ex. U-235, U-238)

ISOTOPESEx. CARBON with an atomic mass equal to 12 or

carbon-12, is the most common form of carbon.Carbon-12 = 6 protons + 6 neutronsCarbon-14 = 6 protons + 8 neutronsDIFFERENT NUMBER OF NEUTRONS =

ISOTOPE 0F CARBON

AVERAGE ATOMIC MASSThe average atomic mass of an element is the

weighted-average mass of the mixture of its isotopes.

NOTE—the average atomic mass of an element is close to the mass of its most common isotope.

ATOMIC MASS = AVERAGE = MOST COMMON

CHAPTER 18—PROPERTIES OF ATOMS AND THE PERIODIC TABLE

Section 3—THE PERIODIC TABLE

Dimitri Mendeleev—THE ORIGINAL PERIODIC TABLE

LATE 1800’S—RUSSIAN CHEMIST

PERIODIC TABLE—BASED ON INCREASING ATOMIC MASSES

THE ORIGINAL PERIODIC TABLEIn the late 1800’s, Dmitri Mendeleev, a Russian

chemist, searched for a way to organize the elements in order of increasing atomic masses.

On Mendeleev’s table, the atomic mass gradually INCREASES from left to right.

THE MODERN PERIODIC TABLEIf you look at the modern periodic table, you will

see several examples, such as COBALT and NICKEL, where the atomic masses DECREASE from left to right.

In 1913, the work of Henry G.J. Mosely, a young English scientist, led to the arrangement of elements based on their INCREASING atomic numbers instead of atomic masses.

REGIONS OF THE PERIODIC TABLE--PERIODS

The horizontal rows are of elements are called periods. (1-7)

The elements INCREASE by 1 PROTON and 1 ELECTRON as you go from left to right in a period.

REGIONS OF THE PERIODIC TABLE--METALS

All elements in blue are METALS—EXS. Iron, zinc, copper

Most are SOLIDSALSO:SHINYDRAWN INTO WIRES

(DUCTILE)POUNDED INTO SHEETS

(MALLEABLE)GOOD CONDUCTORS OF

HEAT AND ELECTRICITY

REGIONS OF THE PERIODIC TABLE--NONMETALS

All elements in yellow are NONMETALS—EXS. Oxygen, bromine, carbon

Most are GASESBRITTLEPOOR CONDUCTORS OF HEAT AND

ELECTRICITYThose elements in green are METALLOIDS—EXS.

Boron, silicon---properties of both METALS AND NONMETALS

REGIONS OF THE PERIODIC TABLE--GROUPS

• Vertical columns in the periodic table are called GROUPS or FAMILIES, and are numbered 1 through 18.

• Elements in each group have similar PROPERTIES.

• GROUP 11—Copper, silver, gold—All shiny metals; All good conductors of electricity and heat.

What is responsible for the similar properties?

ELECTRONS

Electrons are located in the electron cloud.

Electrons have different amounts of ENERGY within the electron cloud.

↑ ENERGY = ↑DISTANCE from the nucleus

ENERGY LEVELS = PERIODS = SHELLS

These ENERGY LEVELS (PERIODS) are named using numbers 1-7.

ENERGY LEVEL 1 or PERIOD 1—may acquire 2; max number of electrons is 2.

ENERGY LEVEL 2 or PERIOD 2—may acquire 8; max number of electrons is 10 ( 2 from period 1 and 8 from period 2).

ENERGY LEVEL 3 or PERIOD 3—may acquire 8; max number of electrons is 18 (2 from period 1, 8 from period 2, and 8 from period 3).

ATOMIC NUMBER = PROTONS = ELECTRONS

REMEMBER—that the atomic number is EQUAL to the number of protons in an atom.

ALSO—the number of protons is EQUAL to the number of electrons in an atom.

OUTER ENERGY LEVEL ELECTRONSElements in the same GROUP have the same number of

electrons in their outer energy level. (SAME PROPERTIES)

These OUTER ELECTRONS are so important in determining the CHEMICAL PROPERTIES of an element that a special way to represent them has been developed.

ELECTRON DOT DIAGRAMSAn electron dot diagram uses the symbol of the

element and dots to represent the electrons in the OUTER ENERGY LEVEL.

ELECTRON DOT DIAGRAMS are also used to show how the electrons in the OUTER ENERGY LEVEL are bonded when elements combine to form COMPOUNDS.

LONG VERSION—hydrogen, helium—period-1,energy level-1 = 1 ring; hydrogen—atomic number 1= 1 electron; helium--atomic number 2 =2 electrons

LONG VERSION—lithium, neon—period-2,energy level-2 = 2 rings; lithium—atomic number 3= 3 electrons—max 2 in 1st , 1 in 2nd;

neon--atomic number 10 =10 electrons—max 2 in 1st, max 8 in 2nd.

LONG VERSION—sodium, argon--period-3,energy level-3 = 3 rings; sodium—atomic number 11=11 electrons—max 2 in 1st , max 8 in 2nd,

and 1 in 3rd; argon--atomic number 18=18 electrons—max 2 in 1st, max 8 in 2nd, and max 8 in 3rd.

SHORT-CUT—The elements in GROUP-1 have 1 electron in their OUTER ENERGY LEVEL. This electron dot

diagram represents that one electron.

SHORT-CUT—The elements in GROUP-17, the HALOGENS, have 7 electrons in their OUTER ENERGY

LEVEL.

BONDING—Require complete OUTER ENERGY LEVELS

A common property of the halogens, Group-17, is the ability to form compounds with elements from Group-1.

Group-1 element, SODIUM, 1e, reacts with Group-17 element,CHLORINE, 7e. (1 + 7=8)

NOT ALL ELEMENTS WILL COMBINE READILY WITH OTHER ELEMENTS.

The elements in Group-18 have COMPLETE OUTER ENERGY LEVELS.

This makes Group-18, the NOBLE GASES, relatively unreactive.

SHORT-CUT—The elements in GROUP-18, the NOBLE GASES, have 8 electrons in their OUTER ENERGY

LEVEL.