Doping mens introduction of small amount of impurities like phosphorus , arsenic or boron into the pure crystal . In pure silicon , ther are four valenices used in bonding with other four adjacent silicon crystal is doped with a group -15 element ( with five valence electron ) such as P, As , or Bi , the structure of the crystal lattic remains unchanged . Out of the five valence electron of group -15 doped element four element are used in normal covalent bonding with silicon while fiffth electron is delcoasiled and thus conducts electricity
Doping a silicon crystal with a group -13 element (with three valence electrons ) such as B, Al, Ga or In products a semiconductor with three electrons in in dopant . The place where fourth electron is missing is called an electron vacancy or hole . Such hole can move throught the crystal like a positive charge giving rise conduction of electricity.
No. of valene electrons in silicon are :

Options

A. 3
B. 4
C. 5
D. 6

Answer

To solve the question regarding the number of valence electrons in silicon and the effects of doping with group-15 and group-13 elements, we can break it down step by step.

### Step-by-Step Solution:

1. **Identify the Valence Electrons in Silicon:**
- Silicon (Si) is in group 14 of the periodic table.
- Each silicon atom has 4 valence electrons. This is because the group number indicates the number of valence electrons for main group elements.

2. **Understanding Doping with Group-15 Elements:**
- When silicon is doped with a group-15 element (like phosphorus, arsenic, or bismuth), these elements have 5 valence electrons.
- In this case, 4 of the 5 valence electrons from the dopant will form covalent bonds with the surrounding silicon atoms, just like the silicon atoms do.

3. **Effect of the Fifth Electron:**
- The fifth electron from the group-15 element does not participate in bonding and becomes delocalized.
- This delocalized electron contributes to electrical conductivity, making the silicon a better conductor.

4. **Understanding Doping with Group-13 Elements:**
- When silicon is doped with a group-13 element (like boron, aluminum, gallium, or indium), these elements have 3 valence electrons.
- In this scenario, the group-13 element can only form 3 bonds with the surrounding silicon atoms.

5. **Creation of Holes:**
- Since silicon has 4 valence electrons, the absence of one electron (due to the 3 electrons from the group-13 element forming bonds) creates a vacancy known as a "hole."
- These holes can move through the crystal lattice and act as positive charge carriers, contributing to electrical conduction.

### Final Answer:
The number of valence electrons in silicon is **4**.