Which of the following statements are correct?

Options

A. Helium has the highest first ionisation enthalpy in the periods table.
B. Chlorine has less negative electron gain enthalpy than fluorine.
C. Mercury and bromine are liquids at room temperature.
D. In any period, atomic radius of alkali metal is the highest.

Explanation

To determine which of the given statements are correct, we will analyze each statement one by one.

### Step-by-step Solution:

1. **Statement 1: Helium has the highest first ionization enthalpy in the periodic table.**
- **Analysis:** Ionization enthalpy is the energy required to remove the most loosely bound electron from an atom in its gaseous state. Helium has a configuration of 1s², meaning it has two electrons in the first shell, which is very close to the nucleus. Due to its fully filled configuration and the strong attraction between the nucleus and the electrons, helium indeed has the highest first ionization enthalpy in the periodic table.
- **Conclusion:** This statement is **correct**.

2. **Statement 2: Chlorine has less negative electron gain enthalpy than fluorine.**
- **Analysis:** Electron gain enthalpy is the energy released when an atom gains an electron. Fluorine is smaller in size compared to chlorine, and it has a very high electron gain enthalpy (more negative) because the incoming electron experiences less repulsion in the small 2p subshell. Chlorine, being larger, has a less negative electron gain enthalpy because the incoming electron would feel more repulsion in the larger 3p subshell. Therefore, chlorine has more negative electron gain enthalpy than fluorine.
- **Conclusion:** This statement is **incorrect**.

3. **Statement 3: Mercury and bromine are liquids at room temperature.**
- **Analysis:** At room temperature, mercury (Hg) is a liquid due to its unique properties, and bromine (Br) is also a liquid. Both elements exist in liquid form under standard conditions.
- **Conclusion:** This statement is **correct**.

4. **Statement 4: In any period, atomic radius of alkali metal is the highest.**
- **Analysis:** Alkali metals are located in Group 1 of the periodic table. As we move from left to right across a period, the effective nuclear charge increases, causing the atomic radius to decrease. However, alkali metals, being on the far left of a period, have the largest atomic radius compared to other elements in the same period.
- **Conclusion:** This statement is **correct**.

### Final Conclusion:
The correct statements are:
- Statement 1: Correct
- Statement 2: Incorrect
- Statement 3: Correct
- Statement 4: Correct

Thus, the correct options are **1, 3, and 4**.

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