PERIODIC PROPERTIES : atomic size, ionisation enthalpy

Periodic Properties:

Atomic Radius
It is the distance from the centre of the nucleus to the outermost shell of electrons. Covalent
radius for an atom A in a
 molecule.

In general, the atomic size decreases on moving from left to right in a period due to increase in
effective nuclear charge and increases on moving from top to bottom in a group due to addition
of new shells.

van der Waals’ Radius
It is defined as one-half of the distance between the nuclei of two non – bonded isolated atoms
or two adjacent atoms belonging to two neighbOuring molecules of an element in the solid
state.
Metallic Radius
It is define as one-half of the distance between the centres of nuclei of the two adjacent atoms
in the metallic crystal.
onic Radius
An atom can be changed to a cation by losing of electrons and to an anion by gain of electrons.
A cation is always smaller than the parent atom because during its formation effective nuclear
charge increases and sometimes a shell may also decrease. On the other hand, the size of an
anton is always larger than the parent atom because during its formation effective nuclear
charge decreases.
In case of isoelectronic ions, r he higher the nuclear charge. smaller is the size. e.g., AI
3
<
Mg
2+
< Na
+
< f
-< O
2-< N
3-
covalent radius < metallic radius < van der Waals‟ radius

Ionisation Enthalpy (IE)
It is the amount of energy required to remove the loosely bound electron from the isol~ted
gaseous atom.
A(g) + IE → A
+
(g) + e
-Various factors with which IE depends

(i) Atomic size. varies inversely
(ii) Screening effect: varies inversely
(iii) Nuclear charge: varies directly
Generally left to right in periods ionisation enthalpy increases; down the group, it decreases.
IE values of inert gases are exceptionally higher due to stable configuration.
Successive ionisation enthalpies
IE
3 > IE
2 > IE
1
IE
1
of N is greater than that of oxygen due to stable half – filled 2p-orbitals.