Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) : Metallic character decreases and non-metallic character increases on moving from left to right in a period.
Reason (R): It is due to increase in ionisation enthalpy and decrease in electron gain enthalpy, when one moves from left to right in a period.
In the light of the above statements, choose the most appropriate answer from the options given below:

We have to judge each statement separately and then see whether the Reason really explains the Assertion.

First let us look at the trend of metallic and non-metallic character within one period of the modern periodic table. On moving from the extreme left (alkali metals) to the extreme right (halogens and noble gases) the effective nuclear charge $$Z_{\text{eff}}$$ experienced by the outer electrons keeps on increasing because the atomic number increases but the additional electrons enter the same valence shell. Due to this higher $$Z_{\text{eff}}$$ the attraction between the nucleus and the valence electrons becomes stronger. A stronger attraction naturally makes it harder for an atom to lose electrons; therefore the metallic character, which is the tendency to lose electrons and form positive ions, decreases. Simultaneously, the stronger nuclear pull favours the gain of electrons; thus the non-metallic character, which is the tendency to gain electrons and form negative ions, increases. Hence the statement

$$\text{Metallic character} \downarrow \quad\text{and}\quad \text{Non-metallic character} \uparrow\quad \text{from left to right in a period}$$

is correct. So, Assertion (A) is true.

Now we examine the Reason (R), which cites two energetic quantities:

(i) Ionisation enthalpy $$\Delta_{\text{i}}H$$ : the energy required to remove one mole of electrons from one mole of gaseous atoms.

(ii) Electron gain enthalpy (electron affinity) $$\Delta_{\text{eg}}H$$ : the enthalpy change when one mole of electrons is added to one mole of gaseous atoms.

Across a period, because $$Z_{\text{eff}}$$ increases, the outer electrons are held more tightly, so

$$\Delta_{\text{i}}H \uparrow \quad \bigl(\text{ionisation enthalpy increases}\bigr).$$

For electron gain enthalpy the convention is that a more negative value indicates a greater tendency to accept an electron. Across a period the attraction for an extra electron normally becomes stronger; thus the magnitude of $$\Delta_{\text{eg}}H$$ increases, making the value more negative. In words, electron gain enthalpy becomes more negative, or its magnitude increases; it does not decrease. Therefore the statement in the Reason

“... and decrease in electron gain enthalpy”

is incorrect because the trend is actually the opposite. So, Reason (R) is false.

Since the Assertion is correct but the Reason is wrong, the Reason cannot possibly be the correct explanation of the Assertion.

Hence, the correct answer is Option D.