Which of the following element has highest shielding constant ?

Options

A. Mg
B. Al
C. K
D. Ga

Answer

To determine which element has the highest shielding constant among magnesium, aluminum, potassium, and gallium, we will follow these steps:

### Step 1: Understand Shielding Constant
The shielding constant refers to the extent to which inner electrons reduce the effective nuclear charge felt by outer electrons. The more inner electrons present, the greater the shielding effect.

### Step 2: Identify the Atomic Numbers and Electronic Configurations
We need to find the atomic numbers and electronic configurations for each of the elements:
- **Magnesium (Mg)**: Atomic number = 12
- Electronic configuration: 1s² 2s² 2p⁶ 3s²
- **Aluminum (Al)**: Atomic number = 13
- Electronic configuration: 1s² 2s² 2p⁶ 3s² 3p¹
- **Potassium (K)**: Atomic number = 19
- Electronic configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹
- **Gallium (Ga)**: Atomic number = 31
- Electronic configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p¹

### Step 3: Count the Inner Electrons
Next, we will count the number of inner electrons for each element:
- **Magnesium**:
- Inner electrons = 10 (1s² 2s² 2p⁶)
- **Aluminum**:
- Inner electrons = 10 (1s² 2s² 2p⁶)
- **Potassium**:
- Inner electrons = 18 (1s² 2s² 2p⁶ 3s² 3p⁶)
- **Gallium**:
- Inner electrons = 28 (1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s²)

### Step 4: Determine the Element with the Highest Shielding Constant
From the counts of inner electrons:
- Magnesium: 10 inner electrons
- Aluminum: 10 inner electrons
- Potassium: 18 inner electrons
- Gallium: 28 inner electrons

Gallium has the highest number of inner electrons (28), which means it has the highest shielding constant.

### Conclusion
The element with the highest shielding constant is **Gallium (Ga)**.

---