- Draw the Lewis structure of HCN.HCN. Include lone pairs.
- Draw the Lewis structure of CO2-3.CO32-. Include lone pairs. Formal charges are optional.
- What is the number of lone pair(s) on the central atom in POCI3? (Pis the least electronegative atom)
The Correct Answer and Explanation is :
1. Lewis Structure of HCN
The molecule HCN consists of hydrogen (H), carbon (C), and nitrogen (N). Carbon is the central atom because it is the least electronegative atom after hydrogen.
Steps to draw the structure:
- Calculate the total number of valence electrons:
- H: 1
- C: 4
- N: 5
Total = ( 1 + 4 + 5 = 10 ) electrons.
- Arrange the atoms: H–C–N.
- Place single bonds between H and C, and C and N.
- Distribute the remaining electrons to satisfy the octet rule for C and N.
- To satisfy the octet rule for carbon, form a triple bond between C and N.
Structure:
H:C:::N:
- Lone pairs: Nitrogen has one lone pair.
2. Lewis Structure of ( \text{CO}_3^{2-} ) (Carbonate Ion)
Steps to draw the structure:
- Calculate the total number of valence electrons:
- C: 4
- O: ( 3 \times 6 = 18 )
- ( 2- ) charge: +2
Total = ( 4 + 18 + 2 = 24 ) electrons.
- Arrange the atoms: O–C–O–O.
- Place single bonds between C and the three O atoms.
- Distribute the electrons around O atoms to complete their octets.
- Place remaining electrons around C, then form a double bond with one O atom for proper charge distribution.
Structure:
O
║
O–C–O
- Lone pairs: Each single-bonded oxygen has three lone pairs, and the double-bonded oxygen has two lone pairs. Formal charges are: -1 for two single-bonded oxygens and 0 for carbon and the double-bonded oxygen.
3. Lone Pairs on the Central Atom in ( \text{POCl}_3 )
Answer: The central atom (P) has 0 lone pairs.
Explanation:
- Valence electrons of P: Phosphorus has 5 valence electrons.
- Bonding in ( \text{POCl}_3 ):
- P forms single bonds with three chlorine atoms (each contributing 1 electron).
- P forms a double bond with oxygen (O contributes 2 electrons).
- Electron distribution: All 5 valence electrons of P are involved in bonding, leaving no lone pairs.
Phosphorus expands its octet to accommodate more bonds, which is possible because it has d-orbitals in the third period of the periodic table. Hence, all electrons on P are used in bonding, leaving 0 lone pairs.