PERIODIC TREND CONTENT

WEEK TWO
TOPIC: THE PERIODIC TRENDS
CONTENT
 The Periodic Properties of Elements: Atomic size, Ionic size,
Ionization Energy, Electron Affinity, Electronegativity down the group
and across the period.
 Diagonal Relationship in the Properties of Elements in the Periodic
Table.

PERIODICITY
Periodicity with respect to the periodic table is defined as the
variation in the properties of elements in a regular pattern both
down the groups and across the periods.

REASONS FOR PERIODIC VARIATION IN PROPERTIES
Two opposing factors are responsible for the variation in properties of
elements. These are
1. Effect of increasing positive nuclear charge.
2. Screening effect of inner shell electrons.
EFFECT OF NUCLEAR CHARGE AND INNER SHELL ELECTRONS
1. Nuclear charge exerts force of attraction on the electrons towards the
nucleus. This force of attraction increases as a result of increasing
nuclear charge across periods.
2. Inner shell electrons screen the nuclear charge from exerting its force
of attraction outwards to outer electrons. Screening effect is constant
across each period but increase down the groups.
The overall effects of these are:
a. Effect of increasing nuclear charge overpowers the screening
effect of
inner shell electrons across a period.
b. Screening effect of inner shell electrons overpowers the effect of
increasing nuclear charge down the groups.
These effects are the reasons for periodic variations in properties of
elements.

ATOMIC PHYSICAL PROPERTIES OF ELEMENTS THAT SHOW
PERIODIC VARIATION
1. ATOMIC RADIUS: Atomic radius is one half the distances between
two covalently bonded atoms. For electrovalent compounds, ionic

, radius is a measure of the distance between the centre of the ion and
the centre of its nearest neighbour of opposite charge.
2. IONIZATION ENERGY: Ionization energy is the energy required to
remove one mole of electrons from one mole of gaseous atom to
produce one mole of gaseous ions.
3. ELECTRONEGATIVITY: Electronegativity is the power of an atom of
an element to attract electrons to become negatively charged.
4. ELECTROPOSITIVITY: Electropositivity is the power of the atom of an
element to lose electron and become positively charged.
5. ELECTRON AFFINITY: Electron affinity is the energy change which
accompanies the addition of one mole of electrons to one mole of
gaseous atom of an element to form negatively charge ions.

EVALUATION
1. State five atomic physical properties of elements.
2. Define two of the stated atomic physical properties

TRENDS IN ATOMIC PROPERTIES
1. Atomic radius decreases across the periods from left to right while it
increases down the groups from top to bottom.
2. Ionization energy increases across the periods left to right while it
decreases down the groups from top to the bottom.
3. Electronegativity increases from left to right across each period while
it decreases down the groups from top to bottom.
4. Electropositivity decreases from left to right across the period while it
increases down the groups from top to bottom.
5. Electron affinity increases from left to right across each period while it
decreases down the groups. This is because atoms with smaller
atomic radii tend to have a stronger attraction for electrons and so form
negative ions more readily.
6. Ionic radius decreases for metal across the periods and increase for non
– metals.
Ionic radius generally increases down the groups and decreases across the
periods.

NOTE
Cations formed by electron loss are smaller in size than the neutral atom
from which they are formed and anions formed by electron gain are larger
in size than the neutral atoms that form them. Also, the larger the charge