Shapes of molecules — common mistakes

When asked to explain a shape or bond angle, marks are awarded for specific points. Always state: (1) the number of bonding pairs on the central atom, (2) the number of lone pairs on the central atom, (3) the resulting molecular shape (e.g., trigonal pyramidal), and (4) if explaining a reduced bond angle, explicitly state that 'lone pair-bonding pair repulsion is greater than bonding pair-bonding pair repulsion'.

For the purpose of determining molecular shape, a double or triple bond is treated as a single region of electron density, sometimes called an 'electron domain'. In CO₂, the central carbon atom has two double bonds (O=C=O). We count this as two electron domains. Two domains will arrange themselves linearly to be as far apart as possible, resulting in a 180° bond angle and a linear molecule.

The valence electrons are the outermost electrons and are the ones involved in bonding and forming lone pairs. The inner-shell (core) electrons are held tightly to the nucleus and do not influence the molecular geometry, so they can be ignored when applying VSEPR theory.

VSEPR is a simple model that is remarkably effective for predicting the shapes of a vast number of molecules. However, it is a model, not a perfect law. For some very large or complex molecules, or those with heavy central atoms, other factors can come into play, and the predictions may be less accurate. For A-Level Chemistry, VSEPR theory is the primary tool and is considered accurate for the molecules you will encounter.