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	Z − 1	-	-
B	Z	-	-
C	Z + 1	-	-

Also, explain the following:

1. Electron affinity of element A is more than that of element C.
2. lonization energy of element C is less than that of element A.
3. 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	Z − 1	7	17
B	Z	8	18
C	Z + 1	1	1

1. Element A is a halogen with 7 valence electrons. It needs only 1 electron to achieve a stable octet, releasing a large amount of energy when it gains an electron. Conversely, element C is an alkali metal with 1 valence electron and readily loses it instead of gaining one. Therefore, element A has a much higher electron affinity.
2. Element C belongs to group 1 and has entered a new principal shell. It has a larger atomic radius and a single outer electron that is highly shielded from the nucleus, meaning it requires very little energy to remove. Element A has a smaller atomic size and a higher effective nuclear charge, holding its valence electrons tightly and requiring far more ionisation energy.
3. Element B is a noble gas with a completely filled valence shell electronic configuration (ns²np⁶). Because its octet is already stable, it has no tendency to accept an extra electron. Forcing an electron into a higher, empty energy level requires adding energy rather than releasing it, resulting in an electron affinity of zero.