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Order of dipole moments of BF3, NH4+, NF3 and NH3 from bond and lone-pair moments

Four species sit in a sample tray between two charged plates. Press start and the bench does the rest: it builds each species in 3D from VSEPR, draws every bond dipole and the lone-pair moment, slides them tip to tail to find the resultant, then switches on the field and reads the torque on the molecule. BF3 and NH4+ do not twist at all, NF3 barely, NH3 strongly - the bars line up in order and the answer is revealed. No prediction stage - input, run, watch, result.

Sub-topics covered

• Bond dipoles from electronegativity differences
• Lone-pair (atomic) dipole on the central atom
• VSEPR shapes: trigonal planar, tetrahedral, trigonal pyramidal
• Vector addition of bond moments; cancellation by symmetry
• NH3 versus NF3: lone pair reinforcing or opposing the bond moments
• Dipole moment of a symmetric ion about its centre
• Torque on a dipole in a uniform electric field, tau = pE sin(theta)

What it teaches

• A bond between atoms of different electronegativity is polar; its dipole points (chemists' crossed arrow) from the delta-plus to the delta-minus atom
• A lone pair on the central atom puts extra negative charge on one side of it and carries a moment of its own, pointing from the atom out into the pair
• The dipole moment of a molecule is the vector sum of its bond and lone-pair moments, so the shape decides whether they add or cancel
• BF3 is trigonal planar (3 bond pairs, no lone pair): three equal B-F moments at 120 degrees sum to exactly zero even though each bond is strongly polar
• NH4+ is tetrahedral (4 bond pairs, no lone pair): four equal N-H moments to the corners of a tetrahedron sum to zero; the ion's charge is symmetric about N so it has no dipole about its centre
• In a trigonal pyramid the sideways parts of the three bond moments cancel and 3 mu_b cos(beta) survives along the axis, with cos^2(beta) = (1 + 2 cos alpha)/3
• In NH3 the N-H moments point toward N, the same way as the lone pair, so they add: 1.47 D measured
• In NF3 the N-F moments point toward F, away from the lone pair, so they nearly cancel: 0.23 D measured, although N-F is the more polar bond

Simulation ID

• ADV-2026-P1-CHE-Q03 · revision 1

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