1) PBr₃
- Phosphorus (P) has 5 valence electrons.
- Each bromine (Br) has 7 valence electrons; three Br atoms contribute 21.
- Total valence electrons: 5 + 21 = 26.
- P is central atom, bonded to three Br atoms with single bonds (using 6 electrons).
- Remaining 20 electrons are placed as lone pairs: 3 lone pairs on each Br (18 electrons) and 1 lone pair on P (2 electrons).
- Structure: P with three single bonds to Br, each Br has three lone pairs, P has one lone pair.
2) N₂H₂
- Two nitrogen atoms each have 5 valence electrons (total 10), two hydrogen atoms each have 1 (total 2).
- Total valence electrons: 12.
- The most stable structure is trans or cis diazene: N=N double bond, each N bonded to one H.
- Each N has one lone pair.
- Structure: H–N=N–H, with a double bond between N atoms, each N has one lone pair.
3) CH₃OH
- Carbon (C) has 4, oxygen (O) has 6, four hydrogens (H) have 1 each (total 4).
- Total valence electrons: 4 + 6 + 4 = 14.
- C is central, bonded to three H and one O. O is bonded to C and one H.
- C–O and O–H are single bonds. C has no lone pairs. O has two lone pairs.
- Structure: H–C–O–H, with two additional H on C; O has two lone pairs.
4) NO₂⁻¹
- Nitrogen (N) has 5, each oxygen (O) has 6 (total 12), plus 1 electron for the negative charge.
- Total valence electrons: 5 + 12 + 1 = 18.
- N is central, bonded to two O atoms.
- Best structure has one N=O double bond and one N–O single bond (with O carrying the negative charge).
- Single-bonded O has three lone pairs (including the extra electron), double-bonded O has two lone pairs, N has one lone pair.
- Resonance exists: both O atoms are equivalent on average.
- Structure: [O=N–O]⁻, with formal charges: double-bonded O (0), single-bonded O (-1), N (+1).
5) C₂H₄
- Each carbon has 4 valence electrons (total 8), four hydrogens have 1 each (total 4).
- Total valence electrons: 12.
- Two C atoms are double-bonded to each other.
- Each C is bonded to two H atoms with single bonds.
- No lone pairs on any atom.
- Structure: H₂C=CH₂, planar geometry.
Parent Tip: Review the logic above to help your child master the concept of lewis structures practice worksheet.
