A. What is the Lewis structure of PCl2F2?
B. Aluminum has 3 valence electrons. A small amount of Aluminum is added to a Silicon crystal. Silicon has 4 valence electrons. Is the resulting semiconductor a p-type semiconductor or an n-type semiconductor? Draw Lewis dot diagram showing the crystal structure (dot diagram shown in class).
The Correct Answer and Explanation is :
A. Lewis Structure of PCl₂F₂
Phosphorus (P) has 5 valence electrons and forms bonds with two chlorine (Cl) atoms and two fluorine (F) atoms.
- Determine total valence electrons:
- Phosphorus: 5 valence electrons
- Chlorine: 7 valence electrons × 2 = 14
- Fluorine: 7 valence electrons × 2 = 14
Total = 5 + 14 + 14 = 33 electrons (or 16 pairs).
- Draw the skeleton structure:
Phosphorus is the central atom with single bonds to the two chlorine atoms and two fluorine atoms. - Distribute remaining electrons:
- Each bond uses 2 electrons (4 bonds = 8 electrons).
- Distribute the remaining electrons (24 electrons) to complete the octets of the outer atoms (Cl and F).
- Verify:
Phosphorus has a complete octet, and all atoms follow the octet rule.
The Lewis structure has phosphorus in the center with single bonds to each Cl and F atom. Each Cl and F atom has 6 non-bonding electrons.
B. Aluminum in Silicon Crystal: p-Type Semiconductor
When aluminum (Al), which has 3 valence electrons, is added to a silicon (Si) crystal (4 valence electrons), it creates a p-type semiconductor:
- Why p-type?
Aluminum has one less valence electron than silicon. This deficiency creates a “hole” (positive charge carrier) in the crystal structure. The hole can accept an electron, enabling conduction by the movement of these holes. - Lewis Dot Diagram:
In the Si crystal, Al substitutes for a Si atom, creating a covalent bond with three Si atoms. However, there is a missing electron in the fourth bond, representing the hole. - Explanation:
A p-type semiconductor results when a dopant with fewer valence electrons than the host material is introduced. These holes behave like positive charge carriers, enhancing conductivity.
In contrast, an n-type semiconductor is created when a dopant with more valence electrons than silicon (e.g., phosphorus) is added, introducing extra electrons for conduction.
Let me know if you’d like a detailed diagram of the crystal structure!