In boron atom shielding of the last electron is due to 

Options

A. electrons of K shell only.
B. all the electrons of K and L shells.
C. two electrons of 1s and 2s each.
D. all the electrons of L shell only.

Answer

To solve the question regarding the shielding of the last electron in a boron atom, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Atomic Number of Boron**:
- The atomic number of boron (B) is 5. This means it has 5 electrons.

2. **Determine the Electronic Configuration**:
- The electronic configuration of boron is 1s² 2s² 2p¹. This indicates that there are 2 electrons in the 1s subshell, 2 electrons in the 2s subshell, and 1 electron in the 2p subshell.

3. **Identify the Last Electron**:
- The last electron in boron is the one in the 2p subshell, which is the outermost electron.

4. **Identify the Shells Involved**:
- The electrons in the K-shell (1s) and L-shell (2s and 2p) are involved in shielding the last electron from the nuclear charge.

5. **Determine Which Electrons Contribute to Shielding**:
- The electrons in the K-shell (1s²) and the electrons in the L-shell (2s²) contribute to the shielding effect. The 2p electron does not shield itself.

6. **Conclusion**:
- The correct answer to the question is that the shielding of the last electron in a boron atom is due to the electrons in the K-shell (1s) and the L-shell (2s).

### Final Answer:
The shielding of the last electron in a boron atom is due to the electrons in the K-shell (1s) and L-shell (2s).

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