Mistake Master
Home› Unit 2 · Compound Structure and Properties› 2.1·2.2·2.3·2.4·2.5·2.6·2.7› Lesson
Skill Check 0 / 10 complete

VSEPR and hybridization

VSEPR turns a flat Lewis structure into a 3-D shape with one idea: electron groups repel and spread as far apart as they can. The trap is confusing all the electron groups with just the atoms you can see.

§1

Domains repel; shape follows.

▸

VSEPR (valence-shell electron-pair repulsion) predicts molecular shape from the Lewis structure. Around the central atom, count electron domains: each bond (single, double, or triple counts once) is one domain, and each lone pair is one domain.

The domains push apart to minimize repulsion, and that arrangement is the electron geometry — two domains linear, three trigonal planar, four tetrahedral, and so on.

The visible molecular shape counts only the atoms, not the lone pairs. Lone pairs still occupy space and repel, so they bend the shape and compress bond angles below the ideal. Water has four domains (tetrahedral electron geometry) but a bent molecular shape, with an angle under 109.5°.

UNIT 2 TOPIC 2.7 • VSEPR AND BOND HYBRIDIZATION GEOMETRY ENGINE Count electron domains first, then name geometry, shape, hybridization, and polarity. DOMAIN COUNT CED ANCHOR · from the Lewis structure Each single, double, or triple bond = ONE domain. Each lone pair = ONE domain. domains = bonds + lone pairs PARENT ELECTRON GEOMETRY 2 LINEAR 180° 3 TRIGONAL PLANAR 120° 4 TETRAHEDRAL 109.5° 5 TRIGONAL BIPYRAMIDAL 90° / 120° 6 OCTAHEDRAL 90° MOLECULAR SHAPES TO RECOGNIZE 2 domains linear 3 domains trigonal planar, bent 4 domains tetrahedral, trigonal pyramidal, bent 5 domains trigonal bipyramidal, seesaw, T-shaped, linear 6 domains octahedral, square pyramidal, square planar EXAMPLES + HYBRIDIZATION CO₂ linear sp BF₃ trigonal planar sp² CH₄ tetrahedral sp³ NH₃ trig. pyramidal sp³ H₂O bent sp³ 5 & 6 domains: shapes only — no d-orbital hybridization SIGMA / PI BONDS single = 1σ double = 1σ + 1π triple = 1σ + 2π π bonds lock rotation, so geometric (cis/trans) isomers become possible. MOLECULAR POLARITY Molecular dipole = vector sum of bond dipoles; a symmetric shape can cancel polar bonds. CO₂ → nonpolar (cancels) H₂O → polar (net dipole) AP Chemistry · Unit 2 · Compound Structure & Properties
Fig. 2.7.1 VSEPR counts electron domains from the Lewis structure: each bond (single, double, or triple) is one domain and each lone pair is one domain. Domains spread to minimize repulsion, setting the electron geometry; lone pairs then bend the visible molecular shape.
§2

From Lewis structure to shape.

▸

Count domains first, then subtract lone pairs to see the shape.

  1. Draw the Lewis structure. Shape prediction starts from a correct Lewis structure, not the flat written formula.
  2. Count domains on the central atom. Bonds (each multiple bond counts as one) plus lone pairs. That count sets the electron geometry.
  3. Name the electron geometry. 2 domains → linear, 3 → trigonal planar, 4 → tetrahedral, 5 → trigonal bipyramidal, 6 → octahedral.
  4. Subtract lone pairs for molecular shape. The shape is the arrangement of atoms only. Lone pairs bend it and squeeze the bond angles below the ideal value.
§3

The pieces you'll meet.

▸

The key distinction is electron geometry versus molecular shape.

domain
Electron domain
A bond (any order) or a lone pair around the central atom.
e⁻ geometry
Electron geometry
The arrangement of all domains that minimizes repulsion.
shape
Molecular shape
The arrangement of atoms only, ignoring lone-pair positions.
lone pair
Lone pair
A non-bonding domain; occupies space and bends the shape.
angle
Bond angle
The angle between bonds; lone pairs compress it below the ideal.
central
Central atom
The atom whose domains are counted to predict the shape.
§4

Worked example: CO₂ versus H₂O.

▸

CO₂. Carbon has two double bonds and no lone pairs: two domains. Two domains spread to 180°, so CO₂ is linear. Electron geometry and molecular shape agree because there are no lone pairs.

H₂O. Oxygen has two bonds and two lone pairs: four domains. Four domains give a tetrahedral electron geometry, but only two of the domains are atoms, so the molecular shape is bent.

The angle. The ideal tetrahedral angle is 109.5°, but water's two lone pairs push harder than bonds and squeeze the H–O–H angle down to about 104.5°.

Lesson. Same 'AB₂' looking formula, opposite shapes — because the domain count and the lone pairs differ. You cannot read shape off the flat formula.

§5

Mistakes that cost real points.

▸
Pitfall · 01

"You can read a molecule's shape straight off its written formula."

The flat formula does not show lone pairs or bond arrangement. CO₂ and H₂O both look like 'central atom with two others,' yet CO₂ is linear and H₂O is bent. Shape comes from the domain count on the central atom, which requires the Lewis structure.

Fix. Draw the Lewis structure, count domains (bonds + lone pairs) on the central atom, and predict from that — never from the bare formula.

Pitfall · 02

"The electron geometry is the same thing as the molecular shape."

Electron geometry counts all domains; molecular shape counts only atoms. When lone pairs are present, the two differ: water's electron geometry is tetrahedral but its molecular shape is bent. Reporting the electron geometry as the shape is a classic error.

Fix. State the electron geometry (all domains) and the molecular shape (atoms only) separately. Subtract the lone pairs to go from one to the other.

Pitfall · 03

"Bond angles are always exactly the ideal value."

Lone pairs repel more strongly than bonding pairs, so they compress the bond angles below the ideal. Water is not 109.5° but about 104.5°; ammonia is about 107°, not 109.5°. Quoting the ideal angle ignores this lone-pair squeeze.

Fix. Start from the ideal angle for the electron geometry, then reduce it for each lone pair present. Lone pairs bend angles inward.

§6

Skill Check.

▸

Ten scenarios. Pick the chips that match your answer, then check. A scenario marks complete the first time every part is right. Progress saves on this device.

0 of 10 scenarios complete