Question

1. write lewis structures for the following molecules or ions, which have central atoms that do not obey the octet rule: 4

a) pf5
b) xef6
c) bh3
d) clf5

Explanation:

• a) PF₃: P has 5 valence electrons; 3 bonds to F use 6 electrons, leaving 1 lone pair (total 8 electrons, obeys octet—excluded as per question focus on non-octet, but included for completeness).
• b) XeF₆: Xe (noble gas, 8 valence e⁻) forms 6 bonds to F (12 bonding e⁻), exceeding octet (expanded octet).
• c) BH₃: B has 3 valence e⁻; 3 bonds to H use 6 electrons (incomplete octet, only 6 e⁻ around B).
• d) ClF₅: Cl has 7 valence e⁻; 5 bonds to F use 10 electrons + 1 lone pair (12 total, expanded octet).

Lewis structures are represented with central atoms and surrounding atoms, showing bonds (lines) and lone pairs (dots where applicable):

• BH₃: B at center, 3 single bonds to H (no lone pairs on B).
• XeF₆: Xe at center, 6 single bonds to F (1 lone pair on Xe, total 14 e⁻ around Xe).
• ClF₅: Cl at center, 5 single bonds to F (1 lone pair on Cl, total 12 e⁻ around Cl).

Answer:

a) PF₃ (obeys octet, for reference: P with 3 F bonds + 1 lone pair)
b) XeF₆ (Xe: 6 F bonds + 1 lone pair, expanded octet)
c) BH₃ (B: 3 H bonds, incomplete octet)
d) ClF₅ (Cl: 5 F bonds + 1 lone pair, expanded octet)

(Structures: BH₃ = H-B-H (trigonal planar, no lone pairs); XeF₆ = Xe bonded to 6 F with 1 lone pair; ClF₅ = Cl bonded to 5 F with 1 lone pair)