The correct reason for smaller size of alkaline earth metals than that of alkali metal is that

Options

A. alkali metals have high value of nuclear charge than alkaline earth metal
B. alkali metals have low value of nuclear charge than alkaline earth metal
C. alkali metal has high value of screening constant than alkaline earth metals
D. Both (1) and (3)

Answer

To determine the reason for the smaller size of alkaline earth metals compared to alkali metals, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Groups**:
- Alkali metals belong to Group 1 of the periodic table (Li, Na, K, Rb, Cs, Fr).
- Alkaline earth metals belong to Group 2 (Be, Mg, Ca, Sr, Ba, Ra).

2. **Understand Atomic Structure**:
- Alkali metals have one electron in their outermost shell (valence shell).
- Alkaline earth metals have two electrons in their outermost shell.

3. **Nuclear Charge**:
- The nuclear charge refers to the total charge of the nucleus, which is determined by the number of protons.
- As we move from Group 1 (alkali metals) to Group 2 (alkaline earth metals), the number of protons increases. This leads to an increase in nuclear charge.

4. **Effect of Increased Nuclear Charge**:
- An increase in nuclear charge results in a stronger attraction between the nucleus and the electrons.
- This stronger attraction pulls the electrons closer to the nucleus, leading to a decrease in atomic size.

5. **Comparison of Sizes**:
- Therefore, alkaline earth metals, which have a higher nuclear charge than alkali metals, will have a smaller atomic size due to the increased effective nuclear charge acting on the outer electrons.

6. **Conclusion**:
- The correct reason for the smaller size of alkaline earth metals compared to alkali metals is that alkaline earth metals have a higher nuclear charge, which results in a stronger attraction to the electrons, pulling them closer to the nucleus and reducing atomic size.

### Final Answer:
The correct reason for the smaller size of alkaline earth metals than that of alkali metals is that alkaline earth metals have a higher nuclear charge than alkali metals.