Which of the following has largest size ?

Options

A. `H^(-)`
B. `He`
C. `Li^(+)`
D. `Be^(+2)`

Answer

To determine which of the given species has the largest size, we will analyze the number of electrons and protons in each option and how these affect the atomic radius.

### Step-by-Step Solution:

1. **Identify the Species and Their Charges**:
- H⁻ (Hydrogen anion)
- He (Helium)
- Li⁺ (Lithium cation)
- Be²⁺ (Beryllium cation)

2. **Determine the Number of Electrons**:
- H⁻: Hydrogen has 1 proton (atomic number 1), but as it is an anion (H⁻), it has gained an extra electron. Therefore, it has **2 electrons**.
- He: Helium has 2 protons (atomic number 2) and thus has **2 electrons**.
- Li⁺: Lithium has 3 protons (atomic number 3), and since it is a cation (Li⁺), it has lost one electron. Therefore, it has **2 electrons**.
- Be²⁺: Beryllium has 4 protons (atomic number 4), and as a cation (Be²⁺), it has lost two electrons. Therefore, it has **2 electrons**.

3. **Determine the Number of Protons**:
- H⁻: 1 proton
- He: 2 protons
- Li⁺: 3 protons
- Be²⁺: 4 protons

4. **Analyze the Attraction Between Protons and Electrons**:
- The atomic radius is influenced by the attraction between the protons in the nucleus and the electrons surrounding it. A higher number of protons means a stronger attraction, which pulls the electrons closer to the nucleus, resulting in a smaller atomic radius.
- H⁻ has 1 proton attracting 2 electrons, which results in the weakest attraction.
- He has 2 protons attracting 2 electrons, resulting in a stronger attraction than H⁻.
- Li⁺ has 3 protons attracting 2 electrons, resulting in an even stronger attraction.
- Be²⁺ has 4 protons attracting 2 electrons, resulting in the strongest attraction.

5. **Conclusion**:
- Since H⁻ has the weakest attraction due to having the least number of protons (1 proton), it will have the largest size among the given options. Therefore, the species with the largest size is **H⁻**.

### Final Answer:
**H⁻ has the largest size.**