According to Aufbau principle , the `19^(th)` electron in an atom goes into the :-

Options

A. (a)4s - orbital
B. (b)3d - orbital
C. (c)4p - orbital
D. (d)3p - orbital

Answer

To determine where the 19th electron in an atom goes according to the Aufbau principle, we can follow these steps:

### Step-by-Step Solution:

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

2. **Determine the Electron Configuration for 18 Electrons**:
- The first 18 electrons fill the orbitals in the following order:
- 1s² (2 electrons)
- 2s² (2 electrons)
- 2p⁶ (6 electrons)
- 3s² (2 electrons)
- 3p⁶ (6 electrons)
- This gives us the electronic configuration: 1s² 2s² 2p⁶ 3s² 3p⁶.

3. **Identify the Next Orbitals to Fill**:
- After filling the first 18 electrons, the next orbitals to consider are 3d and 4s.
- We need to determine which of these orbitals has lower energy.

4. **Apply the n + l Rule**:
- For the 3d orbital:
- n = 3 (principal quantum number)
- l = 2 (azimuthal quantum number for d)
- Therefore, n + l = 3 + 2 = 5.
- For the 4s orbital:
- n = 4
- l = 0 (azimuthal quantum number for s)
- Therefore, n + l = 4 + 0 = 4.

5. **Compare Energies**:
- According to the n + l rule, the lower the value of n + l, the lower the energy of the orbital.
- Since 4s (n + l = 4) has a lower energy than 3d (n + l = 5), the 4s orbital will be filled before the 3d orbital.

6. **Conclusion**:
- Thus, the 19th electron will enter the 4s orbital.

### Final Answer:
The 19th electron in an atom goes into the **4s orbital**.