SCH4U1
Trends in the Periodic Table
Properties of the Elements
Property
atomic number
atomic mass
# of valence electrons:
Definition
The number of protons in an atom.
The number of protons + neutrons.
The number of electrons in the outermost orbit of an atom.
(e.g. fluorine has 7 valence electrons)
metallic character
Metals have characteristically weak force of attraction between the
nucleus and the outermost electrons. The weaker this force, the
greater the metallic character.
atomic radius
The distance from the center of the atom to the outermost electrons.
Atomic radius indicates the size of an atom.
ionization energy
The minimum amount of energy needed to remove the outermost
electron from an isolated atom of a gaseous sample of an element.
electron affinity
The tendency of an atom to attract electrons.
Factors Affecting the Properties
Some of the properties of the elements are related to the force of attraction between
the nucleus and the outermost electrons. This force of attraction is dependent on 2
factors. These are, in order of importance:
a) the distance between the nucleus and the outer electrons
b) the number of protons in the nucleus (the nuclear charge)
Coulomb’s Law describes this electrical force of attraction:
Where:
F is the force of attraction between opposite charges.
q1 is the charge of the nucleus.
q2 is the charge of the outer electron (-1)
r is the atomic radius.
F k
q1q2
r2
Trends in the Periodic Table
a) Groups (Families): The trends in properties from top to bottom are:



atomic number increases
atomic mass increases
# valence electrons is the same
(e.g. Be 2,2 / Mg 2,8,2 / Ca 2,8,8,2 etc.)
The number of valence electrons explains why the groups have similar
chemical properties since they have the same number of electrons to react
with.
Since the distance from the nucleus to the outermost electrons increases due to
the added orbits, the force of attraction between the nucleus and the valence
electrons decreases, explaining these properties:
 atomic radius increases
 metallic character increases
(elements have a weaker hold on electrons)
 ionization energy decreases
(less energy needed to remove electrons since the force of attraction is lower)
 electron affinity decreases (decreasing attraction of electrons by the atom)
b) Periods (rows): The trends in properties from left to right (generally) are:



atomic number increases
atomic mass increases
# of valence electrons increases
The electron orbit (or shell) is the same for all the elements of a period.
Therefore, the nuclear charge is the most significant factor. As the nuclear
charge increases across a period, the force of attraction increases, explaining
these properties:
Since the number of occupied orbits remains the same but the nuclear charge
increases as the atomic number increases across the periods, the force of
attraction between the nucleus and the valence electrons increases.
 metallic character decreases (metals  metalloids  nonmetals)
 atomic radius decreases
(the atom gets smaller as the nuclear charge increases!)
 ionization energy increases
(increase in nuclear charge results in a greater attraction force)
 electron affinity increases (increasing attraction of electrons by atom)
Ionization Energy
Ionization energy is the energy required to remove an electron from the
atom.
First Ionization Energy is the energy to remove the electron furthest
from the nucleus. The second ionization energy removes the second
electron etc.
FIRST IONIZATION (E1)
A + energy
A+ + e-
SECOND IONIZATION (E2) A+ + energy
A2+ + e-
THIRD IONIZATION (E3) A2+ + energy
A3+
+ e-
As each electron is removed, net positive charge increases and it
becomes more difficult to remove electrons. Ionization energy
increases: E3 > E2 > E1
Energy Level Diagram
e.g. Be 1s2 2s2
1s2 2s2
Be
1s2 2s1
Be+
1s2
Be2+
1s1
Be3+
no eBe4+
Note that ionization energies of electrons from the same energy level
has the same order of magnitude.
Ionization Energy Trends
1) Across the Period (Row): Ionization energy increases as the
number of protons increases. Electrons are bound more
tightly and more energy is required to remove them.
2) Down a Group (Column): Ionization energy decreases
because the highest occupied orbital is further from the
nucleus.
First Ionization Energy (kJ/mol)
First I.E. (kJ/mol)
3000
2000
1000
0
0
2
4
6
8
10 12 14 16 18 20
Atomic Number
Exceptions to the Trends
1) Be and B
2) N + O