Question

A, B, C are three elements in which B is an inert gas other than helium. With this information complete the following table.

Element	Atomic number	No. of electrons in the valence shell	Group to which the element belongs
A B C	Z − 1 Z Z + 1	.... .... ....	.... .... ....

Also, explain the following:
(i) Electron affinity of element A is more than that of element C.
(ii) Ionization energy of element C is less than that of element A.
(iii) Electron affinity of B is zero.

Answer 1

Element	Atomic number	No. of electrons in the valence shell	Group to which the element belongs
A B C	Z − 1 Z Z + 1	7 8 1	17 18 1

(i) Electron affinity of elements increases with increase in atomic number and decrease in atomic size. As the size decreases, force of attraction between nucleus and incoming electron increases and hence, the electron affinity increases. Element C contains one extra shell than element A; therefore, element A is smaller in size. Hence, the electron affinity of element A is more than that of element C.

(ii) Ionisation energy of element C is less than that of element A because element C needs to lose only one valence electron to obtain stable noble gas configuration. The energy required to remove one electron from the valance shell of element C is much lesser than the energy required to remove seven electrons from the valence shell of element A.

(iii) Electron affinity of element B is zero because it has a completely filled valence shell; hence, addition of electrons will not take place in element B.