Molecular structure, electron domains and VSEPR

Molecular structure, electron domains and VSEPR

A molecular formula tells you which atoms are present; a Lewis structure and the valence-shell electron-pair repulsion (VSEPR) model help predict how those atoms are arranged. Draw the central atom, bonding pairs and lone pairs first. Electron domains repel, so they spread out. A double bond counts as one domain for shape prediction, but a lone pair repels more strongly than a bonding pair. State both the electron-domain arrangement and the molecular shape when the question requires it. For example, carbon dioxide has two electron domains around carbon and is linear, while water has four domains around oxygen but a bent molecular shape because two domains are lone pairs.

Work it through

For ammonia, nitrogen has three N-H bonding pairs and one lone pair: four electron domains give a tetrahedral electron-domain arrangement. The lone pair pushes the bonding pairs closer together, so the molecular shape is trigonal pyramidal rather than tetrahedral. In a labelled answer, show the lone pair, state the shape and explain the smaller bond angle in one linked sentence.

Mastery target

Draw or interpret a Lewis structure, count electron domains, predict a molecular shape and justify a lone-pair effect using VSEPR.