Given below are two statements : one is labelled as Assertion A and the other is labelled as Reason R :
Assertion A : The first ionisation enthalpy decreases across a period.
Reason R : The increasing nuclear charge outweighs the shielding across the period.
In the light of the above statements, choose the most appropriate from the options given below :

Solution

Chemistry ma’am · 4 steps

Across a period the nucleus pulls harder, so electrons are held tighter and removing one costs more, not less.

1. Check the reason. Across a period, each new electron enters the same shell, and electrons in the same shell shield each other poorly. The nuclear charge rises by one unit at every step, so the effective nuclear charge Z_(eff) increases. R is true.
2. Follow it through. A larger Z_(eff) pulls the outer electrons closer, so the atomic radius shrinks. The outer electron is held more tightly and needs more energy to remove.
3. Check the assertion. So the first ionisation enthalpy generally increases across a period, from about 520 kJ mol⁻¹ for Li to about 2081 kJ mol⁻¹ for Ne. The small dips at B and O do not reverse the overall trend. A is false.
4. So. A is false and R is true. In fact R explains the opposite of A. The answer is (a).

The idea to keep. Across a period, think: same shell, more protons, bigger Z_(eff), smaller atom, higher ionisation enthalpy. Down a group, the new shell wins and ionisation enthalpy falls.

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