Explain why cations are smaller and anions larger in radii than their parent atoms?

Answer

To explain why cations are smaller and anions are larger in radii than their parent atoms, we can break down the reasoning into clear steps:

### Step-by-Step Solution

1. **Definition of Cations and Anions**:
- Cations are positively charged ions formed when an atom loses one or more electrons.
- Anions are negatively charged ions formed when an atom gains one or more electrons.

2. **Effect of Electron Loss in Cations**:
- When an atom loses electrons to form a cation, the number of electrons decreases while the number of protons (positive charge) remains the same.
- This results in an **increased effective nuclear charge** (the net positive charge experienced by electrons).

3. **Increased Nuclear Attraction**:
- The increased effective nuclear charge means that the remaining electrons are pulled closer to the nucleus due to the stronger attraction.
- As a result, the ionic radius of the cation decreases, making it smaller than the parent atom.

4. **Effect of Electron Gain in Anions**:
- When an atom gains electrons to form an anion, the number of electrons increases while the number of protons remains unchanged.
- This leads to a **decreased effective nuclear charge** because the additional electrons increase electron-electron repulsion.

5. **Decreased Nuclear Attraction**:
- The decreased effective nuclear charge means that the nucleus has a weaker pull on the electrons, allowing them to spread out more.
- Consequently, the ionic radius of the anion increases, making it larger than the parent atom.

6. **Conclusion**:
- In summary, cations are smaller than their parent atoms due to increased effective nuclear charge and stronger nuclear attraction after losing electrons, while anions are larger than their parent atoms due to decreased effective nuclear charge and increased electron-electron repulsion after gaining electrons.