The central idea
Start by counting valence electrons and drawing a sensible Lewis structure. Count electron domains around the central atom: a multiple bond is one domain for shape prediction. Lone pairs affect the arrangement but are not named as atoms in the molecular shape.
A worked example
SF₄ has five electron domains: four bonds and one lone pair. Its electron-domain arrangement is trigonal bipyramidal; placing the lone pair equatorially gives a seesaw molecular shape. Bond dipoles do not cancel. XeF₄ has two opposite lone pairs and a square-planar shape, so its symmetric bond dipoles cancel.
A common mistake
A polar bond does not automatically make a polar molecule. Add bond-dipole vectors using the actual geometry.
Try the idea
These original practice questions also appear in the foundation mock. Attempt them before opening the explanation.
Which gas-phase molecule has a permanent dipole moment?
Answer: C
CO₂ is linear, BF₃ is trigonal planar and XeF₄ is square planar; their bond dipoles cancel by symmetry. SF₄ has a seesaw molecular shape because of one equatorial lone pair, so its dipoles do not cancel.
Using the usual molecular orbital description, which species has the highest bond order?
Answer: B
O₂ has bond order 2. Removing one electron from an antibonding π* orbital gives O₂⁺ a bond order of 2.5. Adding one or two antibonding electrons gives O₂⁻ and O₂²⁻ bond orders of 1.5 and 1 respectively.
In the usual molecular orbital model, which species is diamagnetic?
Answer: C
N₂ has all electrons paired and is diamagnetic. O₂ has two unpaired electrons. O₂⁺ and NO have an odd number of electrons and each has an unpaired electron.
Which statements about molecular shape are correct in the usual VSEPR description?
Answer: A, C, D
NH₃ has three bonding pairs and one lone pair, giving a trigonal-pyramidal molecular shape. CO₂ has two electron domains on carbon and is linear, so B is false. XeF₄ has four bonds and two opposite lone pairs in an octahedral electron-domain arrangement. SF₆ has six bonds and no lone pairs on sulfur.
What to do next
Explain your answer aloud, then find a related question in an official paper. If you can only follow a worked solution but cannot start a fresh question, revisit the central idea and try again without the notes.
Apply it to an official paper