CBSE Class 12 Chemistry Notes: Periodic Properties - Periodicity in Atomic Radius and Ionic Radius

For normal elements

(a) In a period from left to right effective nuclear charge increases because the next electron fills in the same shell. So the atomic size decreases. For example the covalent radii of second period elements in Å are as follows –

Li	Be	B	C	N	O	F
1.23	0.89	0.80	0.77	0.74	0.74	0.72

(b)In a group, from top to bottom the number of shells increases. So the atomic size increases. Although the nuclear charge increases but its effect is negligible in comparison to the effect of increasing number of shells. For example the covalent radii of IA group elements in Å are as follows –

Li	Na	K	Rb	Cs
1.23	1.57	2.03	2.16	2.35

The atomic radius of inert gas (zero group) is shown largest in a period because of its Vander Waal’s radius which is generally larger than the covalent radius. The Vander Waal’s radius of inert gases also increases on moving from top to bottom in a group.
For transition elements – There are four series of transition elements

3d – Sc (21) to Zn (30)

4d – Y (39) to Cd (48)

5d – La (57), Hf (72) to Hg (80)

6d – Ac(89), Rf(104) …………. Unb (ununbium) 112 (incomplete)

(a) From left to right in a period

The atomic size first decreases due to the increase in effective nuclear charge and then becomes constant and then increases. In transition elements, electrons are filled in the (n-1)d orbitals. These (n-1)d electrons screen the ns electrons from the nucleus. So the force of attraction between the ns electrons and the nucleus decreases. This effect of (n-1)d electrons over ns electrons is called shielding effect or screening effect. The atomic size increases due to shielding effect and balances the decrease in size due to increase in nuclear charge to about 80%. Thus moving from left to right in a period, there is a very small decrease in size and it may be considered that size almost remains the same. In the first transition series the atomic size slightly decreases from Sc to Mn because effect of effective nuclear charge is stronger than the shielding effect. The atomic size from the Fe to Ni almost remains the same because both the effects balance each other. The atomic size from Cu to Zn slightly increases because shielding effect is more than the effective nuclear charge due to d¹⁰ structure of Cu and Zn. The atomic radii of the elements of 3d transition series are as under.

Sc	Ti	V	Cr	Mn	Fe	Co	Ni	Cu	Zn
1.44	1.32	1.22	1.18	1.17	1.17	1.16	1.16	1.17	1.25

Inner transition elements – As we move along the lanthanide series, there is a decrease in atomic as well as ionic radius. The decrease in size is regular in ions but not so regular in atoms. This is called lanthanide contraction. The atomic radii in Å are as under

La	Ce	Pr	Nd	Pm	Sm	Eu	Gd
1.88	1.82	1.83	1.82	–	1.80	2.04	1.80
Tb	Dy	Ho	Er	Yb	Lu
1.78	1.77	1.76	1.75	1.94	1.73

There are two peaks one at Eu (63) and other at Yb (70). This is due to the difference in metallic bonding. Except Eu and Yb other lanthanides contribute three electrons in metallic bond formation. These two atoms contribute two electrons in the bond formation leaving behind half filled and completely filled 4f-orbitals respectively.

Cause of Lanthanide contraction – In lanthanides the additional electrons enters the (n-2)f orbital. The mutual shielding effect of (n-2) f electrons is very little because the shape of f-subshell is very much diffused. Thus the effective nuclear charge increases then the mutual shielding effect of (n-2)f electrons. The outer electrons are attracted more by the nucleus. Consequently the atomic and ionic radii decreases from La (57) to Lu (71)

This type of contraction also occurs in actinides. The jump in contraction between the consecutive elements in the actinides is greater than lanthanides. This is due to the lesser shielding of 5f-electrons which are therefore pulled more strongly by the nucleus.

(b) In a group

The atomic radius of elements increases moving from first transition series (3d) to second transition series (4d). This is due to the increase in number of shells with the increase in atomic number.

The atomic radii of second (4d) and third (5d) transition series in a group is almost same except Y(39) and La (57). In third transition series, there are fourteen lanthanides in between La (57) of III B and Hf (72) of IV B groups, so the atomic radius of Hf(72) decreases much due to lanthanide contraction in lanthanides. The difference in the nuclear charge in the elements of a group in first and second transition series is + 18 units while this difference in second and third transition series is + 32 units except Y (39) → La(57). Due to the increase of + 32 units in the nuclear charge there is a sizable decrease in the atomic radius which balances the increase in size due to the increase in number of shells.

So in a group moving from second to third transition series, the atomic radii of the elements almost remain the same except IIIB. The difference is about 0.02Å.

Illustration 2. The radius of argon is greater than the radius of chlorine
Solution: In chlorine, the radius means the atomic or covalent radius which is actually half the intermolecular distance between 2 atoms whereas in Argon the radius means the Vander Waal’s radius as Argon is not a diatomic molecule. Vander Waal’s radius is actually half the distance between adjacent molecules. So Vander Waal’s radius being larger than atomic radius, Argon has got a larger radius than chlorine