Which of the following elements shown as pairs with their atomic numbers belong to the same period?

Options

A. Z=19 and Z=38
B. Z=12 and Z=17
C. Z=11 and Z=21
D. Z=16 and Z=35

Explanation

To determine which of the given pairs of elements belong to the same period, we need to analyze their atomic numbers and the corresponding electron configurations. The period of an element is determined by the highest principal quantum number (n) in its electron configuration.

### Step-by-Step Solution:

1. **Identify the pairs of atomic numbers:**
- Pair 1: Z = 19 and Z = 38
- Pair 2: Z = 12 and Z = 17
- Pair 3: Z = 11 and Z = 21
- Pair 4: Z = 16 and Z = 35

2. **Analyze Pair 1 (Z = 19 and Z = 38):**
- Z = 19 corresponds to Potassium (K). The nearest noble gas is Argon (Z = 18). The electron configuration is [Ar] 4s¹, indicating it is in the 4th period.
- Z = 38 corresponds to Strontium (Sr). The nearest noble gas is Krypton (Z = 36). The electron configuration is [Kr] 5s², indicating it is in the 5th period.
- **Conclusion:** They belong to different periods.

3. **Analyze Pair 2 (Z = 12 and Z = 17):**
- Z = 12 corresponds to Magnesium (Mg). The nearest noble gas is Neon (Z = 10). The electron configuration is [Ne] 3s², indicating it is in the 3rd period.
- Z = 17 corresponds to Chlorine (Cl). The nearest noble gas is Neon (Z = 10). The electron configuration is [Ne] 3s² 3p⁵, indicating it is also in the 3rd period.
- **Conclusion:** They belong to the same period.

4. **Analyze Pair 3 (Z = 11 and Z = 21):**
- Z = 11 corresponds to Sodium (Na). The nearest noble gas is Neon (Z = 10). The electron configuration is [Ne] 3s¹, indicating it is in the 3rd period.
- Z = 21 corresponds to Scandium (Sc). The nearest noble gas is Argon (Z = 18). The electron configuration is [Ar] 3d¹ 4s², indicating it is in the 4th period.
- **Conclusion:** They belong to different periods.

5. **Analyze Pair 4 (Z = 16 and Z = 35):**
- Z = 16 corresponds to Sulfur (S). The nearest noble gas is Neon (Z = 10). The electron configuration is [Ne] 3s² 3p⁴, indicating it is in the 3rd period.
- Z = 35 corresponds to Bromine (Br). The nearest noble gas is Krypton (Z = 36). The electron configuration is [Kr] 4s² 3d¹⁰ 4p⁵, indicating it is in the 4th period.
- **Conclusion:** They belong to different periods.

### Final Answer:
The only pair that belongs to the same period is Pair 2: Z = 12 (Magnesium) and Z = 17 (Chlorine).