The first ionization energy of magnesium is smaller as compared to that of elements X and Y, but higher than that of Z. The elements X, Y and Z, respectively, are :

Solution

Chemistry ma’am · 3 steps

Alkali metals fall below Mg, while halogens and noble gases sit well above it.

1. Why Mg is high. Mg has the configuration [Ne]3s², a filled and stable 3s subshell. Removing an electron from it needs a fair amount of energy, about 738 kJ mol⁻¹.
2. Above and below Mg. Alkali metals have one loosely held s electron, so their first ionisation energy is the lowest in each period. Halogens and noble gases have a high effective nuclear charge and hold their electrons much more tightly.

   Element	Outer configuration	First IE (kJ mol⁻¹)	Compared with Mg (738)
   Li	2s¹	520	lower
   Na	3s¹	496	lower
   Cl	3s² 3p⁵	1251	higher
   Ar	3s² 3p⁶	1521	higher
   Ne	2s² 2p⁶	2081	higher

3. So. X and Y must both be higher than Mg, and Z must be lower. Only (b) works: argon and chlorine are above Mg, and sodium is below it.

The idea to keep. Group 1 metals have the lowest first ionisation energy in their period, so any alkali metal falls below Mg. Halogens and noble gases, with high effective nuclear charge, lie well above it.

Answer from the NTA final answer key. Solution by PrepWiser.