Question

Following table shows the successive molar ionisation energy $(kJ mol^{-1})$ of five elements A to E.

Element	Ionisation energy $(kJ mol^{-1})$
1st	2nd
A	2080
B	500
C	740
D	580
E	420

Which two elements are most likely to be in the same group of the periodic table?
A) C and D
B) B and D
C) D and E
D) B and E

Answer 1

The energy per mole required to remove electrons from gaseous atoms or ions is known as molar ionisation energy. The first molar ionisation applies to a neutral atom, the second molar ionisation energy applies to the singly charged ion, the third molar ionisation energy applies to a doubly charged ion and so on.

Complete answer:
Consider the second molar ionisation energies. The second molar ionisation energies of element B and element E increase drastically than that of element A, element C and element D.
This shows that when one electron is removed from element B and element E, they attain some stable configuration.
Thus, element B and element E belong to the same group of the periodic table.
The two main factors that affect the ionisation energy are as follows:
- The force of attraction between the electrons and the nucleus.
- The force of repulsion between the two electrons.

Thus, the correct option is (D) B and E.

Note: In the periodic table, across a period from left to right the molar ionisation energy increases. From left to right, the atomic radius decreases and the electrons are attracted more towards the nucleus. Thus, more energy is required to remove the electron and thus, the molar ionisation energy increases.
In the periodic table, down the group, molar ionisation energy decreases. Down the group, the atomic radius increases and the electrons are repelled from the nucleus. Thus, less energy is required to remove the electron and thus, the molar ionisation energy decreases.