Which pair of atoms form strongest ionic bond ?

Options

A. Al and As
B. Al and N
C. Al and Se
D. Al and O

Explanation

To determine which pair of atoms forms the strongest ionic bond, we need to analyze the given options based on the properties of the elements involved, particularly focusing on their electron affinity and ionic character.

### Step-by-Step Solution:

1. **Identify the Elements in Each Pair**:
- The pairs given are:
- Aluminium-Arsenic
- Aluminium-Nitrogen
- Aluminium-Selenium
- Aluminium-Oxygen

2. **Understand the Concept of Ionic Bonding**:
- Ionic bonds are formed when one atom donates an electron to another atom, resulting in the formation of positive and negative ions. The strength of the ionic bond is influenced by the charge of the ions and the distance between them.

3. **Determine the Common Cation**:
- In all pairs, Aluminium (Al) will act as the common cation, losing three electrons to form Al³⁺.

4. **Analyze the Anions**:
- The anions formed from the non-metals in the pairs will be:
- Arsenic (As) can form As³⁻
- Nitrogen (N) can form N³⁻
- Selenium (Se) can form Se²⁻
- Oxygen (O) can form O²⁻

5. **Evaluate Electron Affinity**:
- Electron affinity is the tendency of an atom to accept an electron. Higher electron affinity indicates a stronger ability to attract electrons, which contributes to the strength of the ionic bond.
- Among the given non-metals, oxygen has the highest electron affinity, followed by nitrogen, selenium, and arsenic.

6. **Consider the Size of the Atoms**:
- The size of the atoms increases down the group in the periodic table. Larger atoms have a lower effective nuclear charge on the outermost electrons, making it harder for them to attract additional electrons.
- Oxygen is smaller than nitrogen, selenium, and arsenic, allowing it to hold onto the extra electron more effectively.

7. **Conclusion**:
- Since Aluminium (Al) forms Al³⁺ and Oxygen (O) forms O²⁻, the resulting ionic compound will be Al₂O₃. The strong ionic bond is primarily due to the high electron affinity of oxygen and the small size of the O²⁻ ion, which allows for a strong electrostatic attraction to the Al³⁺ ion.
- Therefore, the pair that forms the strongest ionic bond is **Aluminium-Oxygen** (Option D).

### Final Answer:
**Aluminium-Oxygen forms the strongest ionic bond.**