Which of the following elements has least number of electrons in its "M" shell ?

Options

A. K
B. Mn
C. Ni
D. Sc

Answer

To determine which of the given elements has the least number of electrons in its "M" shell, we will analyze the electronic configurations of each element provided in the options. The "M" shell corresponds to the third energy level (n=3), which can hold a maximum of 18 electrons (2 in 3s, 6 in 3p, and 10 in 3d).

### Step-by-Step Solution:

1. **Identify the Elements and Their Atomic Numbers**:
- Potassium (K) - Atomic number 19
- Manganese (Mn) - Atomic number 25
- Nickel (Ni) - Atomic number 28
- Scandium (Sc) - Atomic number 21

2. **Write the Electronic Configurations**:
- **Potassium (K)**:
- Atomic number 19:
- Configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹
- **Manganese (Mn)**:
- Atomic number 25:
- Configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁵
- **Nickel (Ni)**:
- Atomic number 28:
- Configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁸
- **Scandium (Sc)**:
- Atomic number 21:
- Configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹

3. **Count the Electrons in the "M" Shell (n=3)**:
- **Potassium (K)**:
- 3s² (2 electrons) + 3p⁶ (6 electrons) = 8 electrons in the M shell.
- **Manganese (Mn)**:
- 3s² (2 electrons) + 3p⁶ (6 electrons) + 3d⁵ (5 electrons) = 13 electrons in the M shell.
- **Nickel (Ni)**:
- 3s² (2 electrons) + 3p⁶ (6 electrons) + 3d⁸ (8 electrons) = 16 electrons in the M shell.
- **Scandium (Sc)**:
- 3s² (2 electrons) + 3p⁶ (6 electrons) + 3d¹ (1 electron) = 9 electrons in the M shell.

4. **Compare the Number of Electrons in the "M" Shell**:
- Potassium (K): 8 electrons
- Manganese (Mn): 13 electrons
- Nickel (Ni): 16 electrons
- Scandium (Sc): 9 electrons

5. **Identify the Element with the Least Number of Electrons in the "M" Shell**:
- The element with the least number of electrons in its "M" shell is **Potassium (K)** with **8 electrons**.

### Final Answer:
The element with the least number of electrons in its "M" shell is **Potassium (K)**.

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