What will be the value of screening constant `(sigma)` for the sodium atom?

Answer

To find the screening constant (σ) for the sodium atom, we can follow these steps:

### Step 1: Determine the Electronic Configuration
The electronic configuration of sodium (Na) is:
- 1s² 2s² 2p⁶ 3s¹

### Step 2: Identify the Electrons in Different Shells
- The outermost shell (n=3) has 1 electron (3s¹).
- The n-1 shell (n=2) has a total of 8 electrons (2s² 2p⁶).
- The inner shell (n=1) has 2 electrons (1s²).

### Step 3: Apply the Screening Constant Formula
The screening constant (σ) can be calculated using the following contributions:
1. For the outermost shell (n=3):
- Number of electrons in the outer shell = 1
- Contribution = (Number of electrons - 1) × 0.35 = (1 - 1) × 0.35 = 0

2. For the n-1 shell (n=2):
- Number of electrons = 8
- Contribution = Number of electrons × 0.85 = 8 × 0.85 = 6.8

3. For the inner shell (n=1):
- Number of electrons = 2
- Contribution = Number of electrons × 1 = 2 × 1 = 2

### Step 4: Calculate the Total Screening Constant
Now, we can sum up the contributions from all shells:
- Total σ = Contribution from outer shell + Contribution from n-1 shell + Contribution from inner shell
- Total σ = 0 + 6.8 + 2 = 8.8

### Final Answer
The value of the screening constant (σ) for the sodium atom is **8.8**.

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