The elements A, B, C and D have atomic numbers 4,12, 17 and 19 respectively. Which pair of elements belong to the same period?

Options

A. B and C
B. A and B
C. A and D
D. C and D

Explanation

To determine which pair of elements belong to the same period, we need to analyze the atomic numbers and their corresponding electronic configurations.

### Step-by-Step Solution:

1. **Identify the Elements and Their Atomic Numbers:**
- Element A: Atomic number 4
- Element B: Atomic number 12
- Element C: Atomic number 17
- Element D: Atomic number 19

2. **Determine the Electronic Configuration:**
- For Element A (Atomic number 4):
- Electronic configuration: 1s² 2s²
- This means A has 2 shells (K and L).
- For Element B (Atomic number 12):
- Electronic configuration: 1s² 2s² 2p⁶ 3s²
- This means B has 3 shells (K, L, and M).
- For Element C (Atomic number 17):
- Electronic configuration: 1s² 2s² 2p⁶ 3s² 3p⁵
- This means C also has 3 shells (K, L, and M).
- For Element D (Atomic number 19):
- Electronic configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²
- This means D has 4 shells (K, L, M, and N).

3. **Determine the Period of Each Element:**
- The period of an element corresponds to the highest principal quantum number (n) in its electronic configuration.
- Element A: Period 2 (n=2)
- Element B: Period 3 (n=3)
- Element C: Period 3 (n=3)
- Element D: Period 4 (n=4)

4. **Identify the Pair of Elements in the Same Period:**
- Elements B and C both belong to Period 3.

### Conclusion:
The pair of elements that belong to the same period is **B (Atomic number 12) and C (Atomic number 17)**.