Shapes of molecules and polarity

The shape of a molecule is set by the electron pairs round its central atom. Bonding pairs and lone pairs repel each other and spread out as far apart as possible: this is electron-pair repulsion theory. Lone pairs repel more strongly than bonding pairs, so they squeeze the bond angles. Whether a molecule is polar depends on both its bonds and its shape.

Key facts

  • Count the electron pairs round the central atom: take its outer electrons (add one for each negative charge, take one away for each positive charge); one is used for each bond, and the rest pair up as lone pairs.
  • 2 pairs: linear, 180°. 3 pairs: trigonal planar, 120°. 4 pairs: tetrahedral, 109.5°. 5 pairs: trigonal bipyramidal, 120° and 90°. 6 pairs: octahedral, 90°.
  • Repulsion: lone pair–lone pair > lone pair–bonding pair > bonding pair–bonding pair. Each lone pair closes the angle by about 2.5°: NH₃ is trigonal pyramidal (107°) and H₂O is bent (104.5°).
  • Name the shape from where the atoms are, not the lone pairs: XeF₄ has 4 bonding pairs and 2 lone pairs and is square planar.
  • A double or triple bond counts as one region of electrons: CO₂ is linear.
  • A bond is polar when the two atoms have different electronegativities: the more electronegative atom is δ−.
  • A molecule is polar if it has polar bonds and its shape does not let the dipoles cancel: H₂O and CHCl₃ are polar; CO₂, BF₃ and CCl₄ have polar bonds but are non-polar.

Worked examples

Example 1

Predict the shape and bond angle of ammonia, NH₃.

  1. Nitrogen is in group 5: 5 outer electrons.
  2. Three of them bond with the three hydrogen atoms: 3 bonding pairs.
  3. The other two make one lone pair. (5 − 3) ÷ 2 = 1
  4. Four pairs point to the corners of a tetrahedron, but the lone pair repels more strongly, closing the angle from 109.5° to about 107°.
  5. Only the atoms are named: a trigonal pyramid.

Answer: trigonal pyramidal, about 107°

Example 2

Predict the shapes of the ammonium ion, NH₄⁺, and of xenon tetrafluoride, XeF₄.

  1. NH₄⁺: nitrogen has 5 outer electrons, and the positive charge takes one away. 5 − 1 = 4
  2. All four bond with hydrogen atoms: 4 bonding pairs, no lone pairs. Tetrahedral, 109.5°.
  3. XeF₄: xenon has 8 outer electrons; 4 bond with fluorine atoms. (8 − 4) ÷ 2 = 2 lone pairs
  4. Six pairs make an octahedron. The two lone pairs go opposite each other, so the four fluorine atoms form a square: square planar, 90°.

Answer: NH₄⁺: tetrahedral, 109.5°. XeF₄: square planar, 90°.

Example 3

Carbon dioxide and water both have polar bonds. Which molecule is polar?

  1. Oxygen is more electronegative than carbon and than hydrogen, so every bond here is polar, with oxygen δ−.
  2. CO₂ is linear: the two C=O dipoles point in opposite directions and cancel. No overall dipole: non-polar.
  3. H₂O is bent (two lone pairs on oxygen): the two O–H dipoles add up to a dipole towards the oxygen. Water is polar.

Answer: Water is polar; carbon dioxide is non-polar.