In potassium, the order of energy levels is

Options

A. `3s gt 3d`
B. `4s lt 3d`
C. `4s gt 4p`
D. `4s =3d`

Explanation

To determine the order of energy levels in potassium, we need to understand the concept of energy levels and the order in which they fill according to the Aufbau principle, Hund's rule, and the Pauli exclusion principle.

### Step-by-Step Solution:

1. **Identify the Atomic Number of Potassium:**
- Potassium (K) has an atomic number of 19. This means it has 19 electrons.

2. **Determine the Electron Configuration:**
- The electron configuration of potassium can be written as follows:
- 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹
- This shows that potassium fills the 4s orbital before the 3d orbital.

3. **Apply the Aufbau Principle:**
- According to the Aufbau principle, electrons fill orbitals starting from the lowest energy level to the highest. The order of filling is generally:
- 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, etc.
- In this case, we see that the 4s orbital is filled before the 3d orbital.

4. **Use the n + l Rule:**
- The n + l rule helps determine the order of energy levels. The value of n is the principal quantum number, and l is the azimuthal quantum number.
- For the orbitals:
- 4s: n = 4, l = 0 → n + l = 4 + 0 = 4
- 3d: n = 3, l = 2 → n + l = 3 + 2 = 5
- Since 4 (for 4s) is less than 5 (for 3d), the 4s orbital is lower in energy than the 3d orbital.

5. **Conclusion:**
- Therefore, the order of energy levels in potassium is:
- 4s < 3d

### Final Answer:
The order of energy levels in potassium is 4s < 3d.