Periodic Properties of P-Block Elements

P-Block element.

The elements in which the last electrons enters In P-sub-shell of the valance shell of their atoms are called as P-block elements.These elements are located at the extreme right of the periodic table. The P-block element has a general valence shell electronic configuration of ns²np^1-6.

The periodicity of the P-block elements.

1. Variation of atomic radii in periodic table.
2. Variation of ionization potential in periodic tble.

[I]Variation of atomic radii in periodic table.

a.Variation in periods.

When we move left to right across the period the atomic size decreased. It is because while moving from left to right across a period, the number of valence shell remain same but due to the addition of +ve charge in the nucleus as well as an extra addition of electron in the same valence shell, force of attraction increases so that contraction takes place and the atomic size decreases.

For example in the second period the atomic size decreased

$$Li>Be>B>C>N>O>F$$

Thus, in a given period, the alkali has the largest and the halogen atom has the smallest atomic size.

b. Variation in groups.

In general, the atomic size increases when we move descending a group from top to bottom because an extra shell of the electron is added in each step from top to bottom. For example, in group IA, the order of atomic size among different elements are as follows,i,e atomic size increased.

$$Li<Na<K<Rb<Cs$$

[II] Variation of ionisation potential in a periodic table.

a.Variation along the period.

In general, Ionisation potential increases with increasing atomic number as a move from left to right along the period. This is because while moving from left to right there is a gradual increase in nuclear charge and a simultaneous decrease in the atomic size and outer electrons get more and more tightly bound to the nucleus. Therefore, it becomes increasingly difficult to remove as electron and hence, the ionisation energy keeps on increasing.

b.Variation along the group.

Ionisation energy decreases regularly in going from top to the bottom in a group. This can be explained on the basis of the following factor.

• On descending down the group, the nuclear charge increased.
• There is a gradual increase in atomic size due to the progressive addition of new energy level.
• The shielding effect of inner shell electrons on valence shell electron gradually increased.

The number in atomic size and the shielding effect overcome the effect of the increased nuclear charge.As the electron in the higher energy level ar less tightly bound to the nucleus, there is gradually decreased in ionisation energy.

Periodic properties related Boron, Aluminium, Gallium, Indium and Thallium.

Atomic radius.

The Atomic radius of these elements does not increase regularly on descending from Boron to Thallium and is irregular.

There is no substantial increase in atomic radius between Al and Galium.Atomic size of Al and the Ga are similar. This is explained as follows.

Electronic configuration of Galium= [Ar] 3d¹⁰4s²4p1

3d¹⁰⁼Penultimate shell.

4s² 4p_¹=valance shell.

Galium hasten d - a Consequently electron in the penultimate shell (3d¹⁰). Since d-orbital have low shielding effect, nuclear attraction on the valence shelConsequently, the valance shell contract and the atomic radius of Galium decreased and become similar to Al.Hence, the similar atomic radius of Ga, Al,is due to the d-block contraction.

Similarly, there is no substantial (high) increase in atomic radius on descending from Indium to Tl.This is explained as follows.

Electronic configuration of Tl= [Xe] 4f¹⁴, 5d¹⁰, 6s² 6p¹.

4f¹⁴=AntiPenultimate shell.

5d¹⁰=Penultimate shell.

6s² 6p¹.=Valance shell.

Tl hasten d-electrons in the penultimate shell (5d¹⁰) and fourteen f-electron in an anti-penultimate shell (4f¹⁴).Since, d and f-orbital have low shielding effect, nuclear attraction on the valance shell increase. Consequently, the valance shell increase and the atomic radius of Tl decrease and become similar to that of In. Hence, a similar atomic radius of Tl to In is due to combined effect of d and f-block contraction.

Ionisation energy.I.E∝1/r

The ionisation energy of these elements do not decrease regularly from boron to thallium and is irregular.

There is no substantial decrease in ionisation energy on decreasing from Aluminium to Galium.i,e ionisation energy of Galium and Aluminium are almost same. This is explained as follows.

Electronic configuration of Galium=[Ar] 3d¹⁰,4s² 4p^1.

^3d10=Penultimate shell.

4s² 4p^1.=Valance shell

Galium has ten-d-electron in a penultimate shell(3d¹⁰). Since d-orbital has low shielding effect, nuclear attraction on the valance shell increased.Consequently, the valance shell contract and the atomic radius of Ga decreased and become similar to Al.Hence, the similar atomic radius of Ga to Al is due to d-block contraction.Since atomic radii of Al and Ga are similar. Their I.E are also similar.

However, I.E increase on descending from Indium to Thallium. This is explained as follows.

Electronic configuration of Thalium= [Xe] 4f¹⁴, 5d¹⁰,6s² 6p¹

^4f14=Antipenultimate shell.

5d¹⁰=penultimate shell.

6s² 6p¹= Valance shell.

Tl hasten d-electrons in the penultimate shell (3d¹⁰) and fourteen f-electron in the antepenultimate shell(4f¹⁴). Since d and f orbitals have low shielding effect, nuclear attraction on the valance shell increased I,e. the valence electrode is strongly held by the nucleus.This increased nuclear attraction on the valance shell due to less shielding effect of d and f orbital which is responsible for higher ionisation of Thallium than Indium.

Reference.

F.A.Cotton and Wilkinson G. Basic inorganic Chemistry. John,Wiley and Sons (Asia), 2007.

Lee., J.D. Concise Inorganic Chemistry. fifth edition. New Delhi: Oxford University Press., 2008.

Sharma, M.L and P.N Chaudhary. A textbook of B.S.C chemistry. Kathmandu Nepal: Ekta Books Thapathali Kathmandu, 2011.