The order of effective nuclear charge is

Options

A. `LI lt Na lt K lt Rb`
B. `LI gt Na gt K gt Rb`
C. `LI lt K lt Na lt Rb `
D. `LI lt Rb lt K lt Na`

Explanation

To determine the order of effective nuclear charge (Z_eff), we need to understand what effective nuclear charge is and how it can be calculated. The effective nuclear charge is the net positive charge experienced by an electron in a multi-electron atom. It can be calculated using the formula:

\[ Z_{eff} = Z - S \]

Where:
- \( Z \) is the atomic number (the total number of protons in the nucleus).
- \( S \) is the shielding constant, which represents the extent to which inner electrons shield the outer electrons from the full nuclear charge.

### Step-by-Step Solution:

1. **Identify the Atomic Number (Z)**:
- For each element in question, find the atomic number which corresponds to the number of protons in the nucleus.

2. **Determine the Shielding Constant (S)**:
- The shielding constant can be estimated based on the number of inner electrons. Generally, for a given electron, the inner electrons will shield some of the nuclear charge. The more inner electrons there are, the greater the shielding effect.

3. **Calculate Effective Nuclear Charge (Z_eff)**:
- Use the formula \( Z_{eff} = Z - S \) to calculate the effective nuclear charge for each element.

4. **Compare the Z_eff Values**:
- Once you have calculated the effective nuclear charge for the elements in question, compare the values to determine the order of effective nuclear charge.

5. **Establish the Order**:
- List the elements in order of increasing or decreasing effective nuclear charge based on your calculations.

### Example:
If we consider elements like Lithium (Li), Beryllium (Be), and Boron (B):
- **Lithium (Li)**:
- \( Z = 3 \), \( S \approx 1 \) (due to 2 inner electrons)
- \( Z_{eff} = 3 - 1 = 2 \)

- **Beryllium (Be)**:
- \( Z = 4 \), \( S \approx 1.6 \) (due to 2 inner electrons)
- \( Z_{eff} = 4 - 1.6 = 2.4 \)

- **Boron (B)**:
- \( Z = 5 \), \( S \approx 1.8 \) (due to 2 inner electrons)
- \( Z_{eff} = 5 - 1.8 = 3.2 \)

### Order of Effective Nuclear Charge:
Based on the calculations:
- Lithium (Li): \( Z_{eff} = 2 \)
- Beryllium (Be): \( Z_{eff} = 2.4 \)
- Boron (B): \( Z_{eff} = 3.2 \)

Thus, the order of effective nuclear charge is:
\[ \text{Li} < \text{Be} < \text{B} \]