The atoms having the bigger size among each of the following pairs are
(i) Mg (At. No .12) or Cl (At. No. 17)
(ii) Na (At. No. 11) or K (At. No. 19)

Options

A. Mg and K
B. Mg and Na
C. Cl and Na
D. Cl and K

Explanation

To determine which atom has a bigger size among the given pairs, we need to understand the trends in atomic size in the periodic table.

### Step-by-Step Solution:

**(i) Comparing Mg (Atomic No. 12) and Cl (Atomic No. 17)**

1. **Identify the Period and Group:**
- Magnesium (Mg) is in Group 2 and Period 3.
- Chlorine (Cl) is in Group 17 and Period 3.

2. **Determine the Electronic Configuration:**
- Mg: 1s² 2s² 2p⁶ 3s² (total of 12 electrons)
- Cl: 1s² 2s² 2p⁶ 3s² 3p⁵ (total of 17 electrons)

3. **Analyze the Position in the Periodic Table:**
- Both Mg and Cl are in the same period (Period 3).
- As we move from left to right across a period, atomic size decreases due to an increase in effective nuclear charge (more protons attract the electrons more strongly).

4. **Conclusion for Pair (i):**
- Since Mg is to the left of Cl in the same period, Mg has a larger atomic size than Cl.

**Answer for (i): Mg has a bigger size than Cl.**

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**(ii) Comparing Na (Atomic No. 11) and K (Atomic No. 19)**

1. **Identify the Period and Group:**
- Sodium (Na) is in Group 1 and Period 3.
- Potassium (K) is in Group 1 and Period 4.

2. **Determine the Electronic Configuration:**
- Na: 1s² 2s² 2p⁶ 3s¹ (total of 11 electrons)
- K: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ (total of 19 electrons)

3. **Analyze the Position in the Periodic Table:**
- Na is in Period 3 and K is in Period 4.
- As we move down a group (from Na to K), atomic size increases because additional electron shells are added, which outweighs the increase in nuclear charge.

4. **Conclusion for Pair (ii):**
- Since K is in a lower period than Na, K has a larger atomic size than Na.

**Answer for (ii): K has a bigger size than Na.**

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### Final Answers:
- (i) Mg has a bigger size than Cl.
- (ii) K has a bigger size than Na.