Which one of the following pairs would have a larger size ? Explain
`(i) K " or " K^(+) (ii) Br " or " Br^(-) (iii) O^(2-) " or " F^(-) (iv) Li^(+) " or " Na^(+) (v) P " or "As (vi) Na^(+) " or " Mg^(2+)`

Explanation

To determine which of the given pairs has a larger size, we need to analyze each pair based on the principles of ionic size and atomic size. Here’s the step-by-step solution:

### Step 1: Analyze K and K⁺
- **K (Potassium)** is a neutral atom, while **K⁺ (Potassium ion)** is a cation.
- Cations are smaller than their parent atoms due to the loss of electrons, which reduces electron-electron repulsion and allows the remaining electrons to be pulled closer to the nucleus.
- **Conclusion:** K > K⁺ (K has a larger size).

### Step 2: Analyze Br and Br⁻
- **Br (Bromine)** is a neutral atom, while **Br⁻ (Bromide ion)** is an anion.
- Anions are larger than their parent atoms because the addition of electrons increases electron-electron repulsion, causing the electron cloud to expand.
- **Conclusion:** Br⁻ > Br (Br⁻ has a larger size).

### Step 3: Analyze O²⁻ and F⁻
- **O²⁻ (Oxide ion)** and **F⁻ (Fluoride ion)** are both anions.
- The size of an anion increases with the number of negative charges. O²⁻ has a greater negative charge than F⁻.
- **Conclusion:** O²⁻ > F⁻ (O²⁻ has a larger size).

### Step 4: Analyze Li⁺ and Na⁺
- **Li⁺ (Lithium ion)** and **Na⁺ (Sodium ion)** are both cations.
- As we move down a group in the periodic table, the size increases due to the addition of new electron shells.
- **Conclusion:** Na⁺ > Li⁺ (Na⁺ has a larger size).

### Step 5: Analyze P and As
- **P (Phosphorus)** and **As (Arsenic)** are in the same group (Group 15).
- Similar to the previous step, the size increases down a group due to the addition of new electron shells.
- **Conclusion:** As > P (As has a larger size).

### Step 6: Analyze Na⁺ and Mg²⁺
- **Na⁺ (Sodium ion)** and **Mg²⁺ (Magnesium ion)** are both cations in the same period (Period 3).
- As the positive charge increases, the size decreases due to increased nuclear charge pulling the electrons closer.
- **Conclusion:** Na⁺ > Mg²⁺ (Na⁺ has a larger size).

### Final Summary of Sizes
1. K > K⁺
2. Br⁻ > Br
3. O²⁻ > F⁻
4. Na⁺ > Li⁺
5. As > P
6. Na⁺ > Mg²⁺