The periodicity is related to the electronic configuration. That is, all chemical and physical properties are a manifestation of the electronic configuration of the elements.
The atomic and ionic radii generally decrease in a period from left to right. As a consequence, the ionization enthalpies generally increase and electron gain enthalpies become more negative across a period. In other words, the ionization enthalpy of the exterme left element in a period is the least and teh electron gain enthalpy of the element on the exterme right is the highest negative. This results into high chemical reactivity at the two extermes and the lowest in he centre. Similarly down the group, the increase in atomic and ionic radii result in gradual decrease in ionization enthalpies and a regular decrease (with exception in some third period elements) in electron gain enthalpies in the case of main group elements.
The loss and gain of electrons can be co-related with the reducting and oxidising behaviour, and also with metallic and non-metallic character respectively, of the elements.
Which of the following statements is correct?

Options

A. ionisation enthalpies of elements decrease along a period and increase along a group in Modern periodic table.
B. In the `3^(rd)` period of Modern periodic table, the two most reactive elements are sodium and fluorine.
C. Fluorine has the least negative electron gain enthalpy among all halogens.
D. Ionisation enthalpy of `Pb` is greater than that of `Sn`.

Explanation

To determine which statement is correct based on the periodicity and electronic configuration, we will analyze each statement step by step.

### Step 1: Analyze the first statement
**Statement:** "Ionization enthalpies of elements decrease along a period and increase along the group."

**Analysis:**
- Ionization enthalpy generally **increases across a period** from left to right due to the increasing nuclear charge, which attracts the electrons more strongly, making them harder to remove.
- Ionization enthalpy generally **decreases down a group** because the atomic size increases, and the outer electrons are farther from the nucleus, making them easier to remove.

**Conclusion:** This statement is **incorrect**.

### Step 2: Analyze the second statement
**Statement:** "In the third period of the modern periodic table, the two most reactive elements are sodium and fluorine."

**Analysis:**
- Sodium (Na) is indeed one of the most reactive elements as it is an alkali metal located at the extreme left of the third period.
- However, the most reactive non-metal in the third period is **chlorine (Cl)**, not fluorine. Fluorine is in the second period.

**Conclusion:** This statement is **incorrect**.

### Step 3: Analyze the third statement
**Statement:** "Fluorine has the least negative electron gain enthalpy among all halogens."

**Analysis:**
- Electron gain enthalpy is the energy released when an electron is added to a neutral atom.
- Among halogens, fluorine has a **more negative electron gain enthalpy** than chlorine due to its small size and high electronegativity. However, the electron gain enthalpy of fluorine is less negative than that of chlorine due to electron-electron repulsion in the small fluorine atom.
- Therefore, **iodine** has the least negative electron gain enthalpy among halogens.

**Conclusion:** This statement is **incorrect**.

### Step 4: Analyze the fourth statement
**Statement:** "The ionization enthalpy of lead (Pb) is greater than that of tin (Sn)."

**Analysis:**
- Both lead and tin belong to the same group (Group 14).
- Ionization enthalpy generally **decreases down a group** due to the increase in atomic size and shielding effect.
- However, due to **lanthanide contraction**, the ionization enthalpy of lead can be higher than that of tin, as the effective nuclear charge felt by the outer electrons in lead can be greater.

**Conclusion:** This statement is **correct**.

### Final Answer:
The correct statement is: **"The ionization enthalpy of lead (Pb) is greater than that of tin (Sn)."**

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