Following statements regarding the periodic trends of chemical reactivity of the alkali metals and the halogens are given. Which of these statements gives the correct picture: A)In alkali metals the reactivity increases but in the halogens it decreases with increase in atomic number down the group B)The reactivity decreases in the alkali metals but increases in the halogens with increases in atomic number down the group. C)In both the alkali metals and the halogen the chemical reactivity decreases with increases in atomic number down the group D)Chemical reactivity increases with increases in atomic number down the group in both the alkali metals and halogens.

Options

A. Least ionisation enthalpy on the left and highest negative electron gain enthalpy on the right.
B. Non-merallic character on the left and metallic character on the right.
C. High atomic radii on the left and small atomic radii on the right.
D. Highest electronegativity on the left and least electronegativity on the right.

Explanation

To solve the question regarding the periodic trends of chemical reactivity of alkali metals and halogens, we need to analyze each statement based on our understanding of periodic trends.

### Step-by-Step Solution:

1. **Understanding Alkali Metals**:
- Alkali metals are located in Group 1 of the periodic table (Li, Na, K, Rb, Cs, Fr).
- As we move down the group, the atomic size increases, and the outermost electron is further from the nucleus. This makes it easier for these metals to lose their outermost electron.
- Therefore, the reactivity of alkali metals **increases** down the group.

2. **Understanding Halogens**:
- Halogens are located in Group 17 of the periodic table (F, Cl, Br, I, At).
- As we move down this group, the atomic size also increases, and the ability to attract electrons (electronegativity) decreases.
- Consequently, the reactivity of halogens **decreases** down the group because they are less able to gain electrons.

3. **Analyzing the Statements**:
- **Statement A**: "In alkali metals the reactivity increases but in the halogens it decreases with increase in atomic number down the group."
- This statement is **correct** based on our analysis.
- **Statement B**: "The reactivity decreases in the alkali metals but increases in the halogens with increases in atomic number down the group."
- This statement is **incorrect**.
- **Statement C**: "In both the alkali metals and the halogens the chemical reactivity decreases with increases in atomic number down the group."
- This statement is **incorrect**.
- **Statement D**: "Chemical reactivity increases with increases in atomic number down the group in both the alkali metals and halogens."
- This statement is **incorrect**.

4. **Conclusion**:
- The only correct statement is **A**. Therefore, the answer to the question is **A**.