Q1(b). Lewis structures of [Br₃]⁻ and thiourea; the most polar bond

Answer in brief

Yes, thiourea has polar bonds; the most polar is N–H (Δχ = 0.84, against 0.49 for C–N and 0.03 for C=S).

Key points

• [Br₃]⁻ has 22 valence electrons; the central Br holds 2 bond pairs and 3 lone pairs (expanded octet), formal charges 0, −1, 0.
• Five pairs round the central Br take a trigonal bipyramidal arrangement with the three lone pairs equatorial, so the ion is linear.
• Thiourea has 24 valence electrons: seven single bonds, a C=S double bond, one lone pair on each N and two on S; every formal charge is zero.
• Pauling electronegativities H 2.20, C 2.55, S 2.58, N 3.04 make N–H the most polar bond; C=S is almost non-polar by electronegativity.
• Nitrogen lone pairs delocalise into the C=S bond, pushing charge onto sulfur, so thiourea has a large dipole moment (about 4.9 D).