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:

Step-by-step solution

Ionization enthalpy is defined as the minimum amount of energy that is required to remove the most loosely bounded electron that is the electron present in the outermost shell, from an isolated gaseous atom. First, ionisation energy increases along the period because effective nuclear charge increases. Along the period Z increases which outweighs the shielding effect. Hence, option C is the answer.