AP Chemistry · Lesson 3

VSEPR, shape and polarity

Know Know it

Count the electron domains on the central atom: each lone pair is one domain, and each bond is one domain whether it is single, double or triple. The domains spread out as far apart as possible (the VSEPR model).

Name the shape from the atoms only. Lone pairs take more room than bonding pairs, so they squeeze the bond angles below the ideal values: about 107° in NH₃ and about 104.5° in H₂O instead of 109.5°.

The number of domains also gives the hybridization: 2 is sp, 3 is sp² and 4 is sp³. Every single bond is a sigma (σ) bond; a double bond is one σ and one pi (π) bond; a triple bond is one σ and two π.

A molecule is polar when it has polar bonds whose pulls do not cancel. Symmetric shapes with identical outer atoms (linear AX₂, trigonal planar AX₃, tetrahedral AX₄, square planar AX₄) are nonpolar. Lone pairs on the central atom or different outer atoms usually leave a net dipole.

Electron domains and shapes
DomainsLone pairsShapeIdeal angle
20Linear180°
30Trigonal planar120°
31BentUnder 120°
40Tetrahedral109.5°
41Trigonal pyramidalUnder 109.5°
42BentUnder 109.5°
50, 1, 2, 3Trigonal bipyramidal, seesaw, T-shaped, linear90° and 120°
60, 1, 2Octahedral, square pyramidal, square planar90°

Worked example Easy

What is the molecular geometry of ammonia, NH₃?

  1. Tetrahedral
  2. Trigonal planar
  3. Bent
  4. Trigonal pyramidalAnswer

How to solve it

  1. Nitrogen has 5 valence electrons: three are in N–H bonds and two form a lone pair.
  2. That is 4 electron domains, arranged tetrahedrally.
  3. Naming the atoms only, the shape is trigonal pyramidal, with H–N–H angles of about 107°.

Why each choice is right or wrong

  • A. Incorrect. Tetrahedral is the arrangement of the four electron domains; the shape names only the atoms, and one domain is a lone pair.
  • B. Incorrect. Trigonal planar needs three domains with no lone pair, but nitrogen has four domains, one of them a lone pair.
  • C. Incorrect. Bent needs two lone pairs on a four-domain atom, as in water; NH₃ has only one lone pair.
  • D. Correct. Nitrogen has three bonds and one lone pair: four domains, so the three H atoms form a pyramid with N at the top.

Trap Electron geometry is not molecular geometry

Water has four electron domains, so its electron geometry is tetrahedral. But two of the four are lone pairs, so the atoms form a bent molecule. Count lone pairs as domains when you work out the arrangement, then leave them out when you name the shape.

Explain Explain it

Task verbs: predict, explain

For an expanded octet, count the electrons left on the central atom after every outer atom has its octet. Each leftover pair is a lone pair. With five domains, lone pairs go in the equatorial positions, where they have the most room.

To decide polarity, say two things: whether the bonds are polar, and whether the shape lets their pulls cancel. Polar bonds alone are not enough to make a polar molecule.

Worked example Medium

What is the molecular geometry of sulfur tetrafluoride, SF₄?

  1. Tetrahedral
  2. Square planar
  3. SeesawAnswer
  4. Trigonal bipyramidal

How to solve it

  1. Valence electrons: 6+4(7)=346 + 4(7) = 34.
  2. Four S–F bonds use 8; three lone pairs on each F use 24; 34−8−24=234 - 8 - 24 = 2 remain, one lone pair on S.
  3. Five domains, one of them a lone pair in an equatorial position: a seesaw.

Why each choice is right or wrong

  • A. Incorrect. Tetrahedral would need four domains, but sulfur has a lone pair in addition to its four bonds, so it has five.
  • B. Incorrect. Square planar needs six domains with two lone pairs, but sulfur has five domains with one lone pair.
  • C. Correct. SF₄ has 34 valence electrons; after four S–F bonds and the fluorine lone pairs, 2 electrons remain on sulfur as one lone pair, so five domains with one lone pair give a seesaw.
  • D. Incorrect. Trigonal bipyramidal is the arrangement of all five electron domains; the shape names only the atoms, and one domain is a lone pair.

Predict Your turn

Carbon dioxide, CO₂, and sulfur dioxide, SO₂, both have two polar bonds to oxygen. In CO₂ the carbon atom has no lone pairs, while in SO₂ the sulfur atom has one lone pair. Predict the shape of each molecule, and predict whether each is polar.

Write it, then compare. Nothing you type here is saved or graded.

Compare with a model answer

A full answer does this

  • Says CO₂ is linear
  • Says CO₂ is nonpolar because its two bond dipoles point in opposite directions and cancel
  • Says SO₂ is bent
  • Says SO₂ is polar because in the bent shape its bond dipoles do not cancel

Model answer

In CO₂, carbon has two electron domains (two double bonds) and no lone pairs, so the molecule is linear. Its two C=O dipoles point in opposite directions and cancel, so CO₂ is nonpolar. In SO₂, sulfur has three domains (two bonds and one lone pair), so the molecule is bent. In a bent shape the two S–O dipoles do not cancel, so SO₂ is polar.

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