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 MODELS
DEMOCRITUS—Greek
philosopher—400 B.C.
THOMSON—English
physicist—1904 (Posorange- p, Neg-red-e)
ATOMIC MODELS
RUTHERFORD—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 CARBON
C = 12,
BECAUSE
 6 PROTONS + 6 NEUTRONS = 12 amu’s
PROTONS IDENTIFY THE
ELEMENT
 The 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 number
Carbon = 6 protons = atomic number is 6
Oxygen = 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
NEUTRONS
 If 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
OR
 NUMBER OF NEUTRONS = BIG - LITTLE
ISOTOPES
 Not 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. U235, U-238)
ISOTOPES
 Ex. CARBON with an atomic mass equal to 12 or
carbon-12, is the most common form of carbon.
 Carbon-12 = 6 protons + 6 neutrons
 Carbon-14 = 6 protons + 8 neutrons
 DIFFERENT NUMBER OF NEUTRONS =
ISOTOPE 0F CARBON
AVERAGE ATOMIC MASS
 The 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 TABLE
 In 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 TABLE
 If 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 SOLIDS
ALSO:
 SHINY
 DRAWN 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 GASES
BRITTLE
POOR CONDUCTORS OF HEAT AND
ELECTRICITY
Those 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
ELECTRONS
 Elements 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 DIAGRAMS
 An 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 level2 = 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 Group1.
 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.