Electronic configuration of multielectron atoms can be written by the use of four quantum numbers and also by following certain principles. Pauli's exclusion principle suggests that maximum capacity of an atomic orbital is two. Auf bau principle suggests that the lower energy orbitals are filled first and hence stability can be attained. Hunds rule of maximum multiplicity suggests that pairing occurs with one electron. The arrangement of electrons in the space around the nucleus can be understood only after writing the electronic configuration. Thus writing electronic configuration is more important in the structure of an atom.
Auf-bau principle is first violated for the element with atomic numbers

Options

A. 21
B. 24
C. 26
D. 29

Answer

To determine the first element that violates the Aufbau principle, we need to analyze the electronic configurations of the given atomic numbers: 21, 24, 26, and 29.

### Step-by-Step Solution:

1. **Understand the Aufbau Principle**: The Aufbau principle states that electrons occupy the lowest energy orbitals first before moving to higher energy orbitals.

2. **Write the Electronic Configuration**: We will write the electronic configurations for each of the given atomic numbers (21, 24, 26, and 29) step by step.

3. **Atomic Number 21 (Scandium)**:
- Configuration:
- 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹
- Total Electrons: 21
- This configuration does not violate the Aufbau principle.

4. **Atomic Number 24 (Chromium)**:
- Configuration:
- 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁴
- Rearranged for stability:
- 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹
- Total Electrons: 24
- This configuration shows that the 3d subshell is half-filled before the 4s subshell is filled, which indicates a violation of the Aufbau principle for stability reasons.

5. **Atomic Number 26 (Iron)**:
- Configuration:
- 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶
- This configuration does not violate the Aufbau principle.

6. **Atomic Number 29 (Copper)**:
- Configuration:
- 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁹
- Rearranged for stability:
- 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d¹⁰
- This configuration also shows a rearrangement for stability but is not the first violation.

### Conclusion:
The first element to violate the Aufbau principle is **Chromium (atomic number 24)**, as it rearranges its electrons to achieve a half-filled 3d subshell for increased stability.