Which of the following statements are correct ?

Options

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

Explanation

To determine which statements are correct regarding the periodic properties of elements, we will analyze each statement step by step.

### Step 1: Analyze the statement about Helium's Ionization Energy
**Statement**: Helium has the highest first ionization enthalpy in the periodic table.

**Explanation**: Ionization energy refers to the energy required to remove an electron from an atom. Helium, being a noble gas with a small atomic radius and a full outer shell, holds its electrons very tightly. As a result, it has the highest ionization energy among all elements. This statement is **correct**.

### Step 2: Analyze the statement about Electron Gain Enthalpy
**Statement**: Chlorine has a less negative electron gain enthalpy than fluorine.

**Explanation**: Electron gain enthalpy is the energy released when an electron is added to an atom. Generally, halogens (Group 17) have high electron gain enthalpies because they are one electron short of a stable configuration. However, fluorine is smaller than chlorine, and while it has a high electron affinity, the electron-electron repulsion in its small size makes it less favorable to gain an additional electron compared to chlorine. Therefore, chlorine has a more negative electron gain enthalpy than fluorine. This statement is **incorrect**.

### Step 3: Analyze the statement about Mercury and Bromine
**Statement**: Mercury and bromine are liquids at room temperature.

**Explanation**: Mercury is the only metal that is liquid at room temperature, and bromine is the only non-metal that is liquid at room temperature. Both of these statements are true. Therefore, this statement is **correct**.

### Step 4: Analyze the statement about Atomic Radius
**Statement**: In any period, the atomic radius is 2 for alkali metals.

**Explanation**: The atomic radius does not have a fixed value of 2 for alkali metals or any other group. The atomic radius generally decreases across a period due to increasing nuclear charge, which pulls electrons closer to the nucleus. Additionally, the atomic radius varies among different elements and is not a constant value. Thus, this statement is **incorrect**.

### Conclusion
Based on the analysis:
- Statement 1 (Helium's ionization energy) is **correct**.
- Statement 2 (Chlorine's electron gain enthalpy) is **incorrect**.
- Statement 3 (Mercury and Bromine as liquids) is **correct**.
- Statement 4 (Atomic radius of alkali metals) is **incorrect**.

Thus, the correct statements are **1 and 3**.

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