Lewis structures & molecular geometry (VSEPR)
The idea
A Lewis structure is an electron inventory: count all valence electrons, connect atoms with bonding pairs, and distribute the remainder as lone pairs until octets are satisfied, using formal charge to choose between candidate arrangements. VSEPR then converts that inventory into three-dimensional shape with a single principle — electron domains (bonds and lone pairs alike) repel and spread out as far as possible. You already handle simple covalent bonding; here you add expanded octets, resonance, and the geometry vocabulary that underlies polarity, reactivity, and spectroscopy.
Keep two geometries separate in your head: the electron-domain geometry counts every domain, while the molecular shape names only the atom positions. Water has four domains (tetrahedral arrangement) but a bent shape, because shape ignores the two lone pairs while the bond angle still feels them. Lone pairs press harder than bonding pairs, compressing angles below the ideal values, and in a trigonal bipyramid they always claim equatorial spots where 90° clashes are fewest.
The persistent misconception is naming the shape from all domains — calling water tetrahedral or ammonia pyramidal-with-four-corners. Train the two-step habit: count domains for the skeleton, then drop the lone pairs from the name but not from the angle reasoning.
Worked example
Determine the Lewis structure, electron-domain geometry, molecular shape, and polarity of SF₄.
- Tally valence electrons: sulfur contributes 6 and each fluorine 7, so 6 + 4 × 7 = 34 electrons to place.
- Build the skeleton with four S–F single bonds (8 electrons), then complete each fluorine octet with three lone pairs (24 electrons). That spends 32, leaving 34 − 32 = 2 electrons, which land on sulfur as a lone pair — an expanded octet of 10, allowed for a period-3 element.
- Count domains on the central atom: four bonds plus one lone pair = five domains, so the electron-domain geometry is trigonal bipyramidal.
- Place the lone pair equatorial: there it meets only two bonds at 90°, versus three if it sat axial, minimizing the strongest repulsions. The atoms then trace a seesaw shape.
- Judge polarity: the seesaw is not symmetric, so the four S–F bond dipoles cannot cancel; SF₄ has a net dipole moment and is polar — consistent with its measured dipole of about 0.6 D.
Answer. SF₄ has 34 valence electrons, a trigonal-bipyramidal domain geometry with an equatorial lone pair, a seesaw molecular shape, and a net dipole — it is polar.
Check your understanding
- Why do lone pairs occupy equatorial rather than axial positions in a trigonal bipyramid, and why does the same question not arise in an octahedron?
- How does formal charge guide you between two Lewis structures that both satisfy the octet rule, and when can it mislead you?
- What would have to change about SF₄ for the molecule to be nonpolar, and which shapes guarantee cancellation of identical bond dipoles?
- How would you explain to a friend why CO₂ is linear but SO₂ is bent, using only electron counting?
Build the foundations first
Lewis structures & molecular geometry (VSEPR) builds on these concepts. If any feel shaky, start there.