Lewis structures, formulas and electron-domain geometry

Lewis structures, formulas and electron-domain geometry

A Lewis structure is a model that represents valence electrons as dots and bonds as shared pairs. It helps distinguish a molecular formula from an arrangement of atoms. Start by counting valence electrons, select a sensible central atom, make single bonds, complete outer atoms and then place any remaining electrons on the central atom. If a question gives a simple molecule such as H₂O, CO₂ or NH₃, use the electron arrangement to predict geometry rather than memorising a shape list. A structural formula shows connections; it does not show the true scale or constant motion of particles.

Work it through

For water, oxygen contributes six valence electrons and two hydrogens contribute one each: eight electrons in total. Two O-H bonds use four electrons, leaving two lone pairs on oxygen. Four electron domains give a tetrahedral electron-domain arrangement; two visible atom positions make the molecule bent.

Mastery target

Construct a basic Lewis structure and use it to move accurately from formula to electron domains to a named shape.