Given below are two statements :
one is labelled as Assertion (A) and the other is labelled as Reaso

Q: Why does metallic character decrease across a period?

As we move from left to right, the effective nuclear charge increases. This pulls the valence electrons closer to the nucleus, making it harder for the atom to lose electrons, which is the defining trait of a metal.

Q: What exactly is electron gain enthalpy?

It is the energy change that occurs when an electron is added to a neutral gaseous atom. For most non-metals, energy is released, so the thermodynamic value is negative.

Q: Why does the reason statement say electron gain enthalpy decreases?

The reason statement is factually incorrect. Across a period, the effective nuclear charge increases, making it easier to add an electron. Thus, the electron gain enthalpy becomes more negative, which means the magnitude of energy released actually increases.

Q: Is ionisation enthalpy the same as ionisation energy?

Yes, for all practical purposes in this context, they refer to the same concept: the energy required to remove the most loosely bound electron from an isolated gaseous atom.

Q: How do I avoid silly mistakes in Assertion-Reason questions?

Always evaluate the Assertion and Reason independently first as true or false statements. Only if both are true should you check if the Reason correctly explains the Assertion. A single incorrect word (like 'decrease' instead of 'increase') makes the entire statement false.

Question

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.

Options:

(A) (A) is false but (R) is true.

(B) (A) is true but (R) is false

(C) Both (A) and (R) are correct and (R) is the correct explanation of (A).

(D) Both (A) and (R) are correct but (R) is not the correct explanation of (A).

Sigma Percentile · Accepted Answer

### The Beauty of Periodic Trends

The periodic table is not just a random arrangement of elements; it is a beautifully orchestrated map of chemical behavior. When we move across a period from left to right, we are essentially taking a journey from the land of metals to the territory of non-metals. Let's break down the physics behind this journey to solve our Assertion-Reason problem.

### Analyzing the Assertion

The Assertion states: **"Metallic character decreases and non-metallic character increases on moving from left to right in a period."**

Think about the extreme left of the periodic table. We have the alkali metals (like Sodium and Potassium). These elements are desperate to lose their single valence electron to achieve a stable noble gas configuration. This tendency to lose electrons is the very definition of **metallic character**.

Now, look at the extreme right (excluding noble gases). We find the halogens (like Fluorine and Chlorine). These elements are one electron short of a stable configuration, making them highly eager to gain an electron. This tendency to gain electrons defines **non-metallic character**.

Therefore, as we travel from left to right, the elements transition from electron-losers to electron-gainers. The metallic character undeniably decreases, and the non-metallic character increases. The Assertion is absolutely **True**.

### Analyzing the Reason

The Reason statement attempts to explain this trend: **"It is due to increase in ionisation enthalpy and decrease in electron gain enthalpy..."**

Let's dissect this into two parts:

**1. Ionisation Enthalpy ($IE$):** This is the energy required to rip an electron away from an atom. As we move from left to right, the number of protons in the nucleus increases, but the new electrons are added to the same principal shell. This results in a stronger effective nuclear charge ($Z_{	ext{eff}}$), which pulls the valence electrons closer and holds them tighter. Consequently, it becomes increasingly difficult to remove an electron. Thus, the ionisation enthalpy **increases**. The first half of the Reason statement is correct.

**2. Electron Gain Enthalpy ($\Delta_{eg}H$):** This is the energy released when an atom gains an electron. Because the effective nuclear charge ($Z_{	ext{eff}}$) is stronger on the right side of the period, these atoms have a much greater ability to attract an incoming electron. When they do, a larger amount of energy is released. In thermodynamic terms, the electron gain enthalpy becomes **more negative**.

When a value becomes more negative, its magnitude is actually *increasing*. The Reason statement claims that the electron gain enthalpy *decreases*. This is a classic trap! The energy released is increasing, not decreasing. Therefore, the second half of the Reason statement is factually **False**.

### The Final Verdict

Since the Assertion is a true statement, but the Reason contains a fatal factual error regarding electron gain enthalpy, the Reason as a whole is false.

This leads us directly to our final answer: **(A) is true but (R) is false.**

Select Answer:

Visualized Solution

$Evaluating the Assertion$

$Evaluating the Reason: Ionisation Enthalpy$

$Evaluating the Reason: Electron Gain Enthalpy$

$Final Conclusion$

The Sigma Insight: Periodic Table and Periodic Properties

The Beauty of Periodic Trends

Analyzing the Assertion

Analyzing the Reason

The Final Verdict

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