Electron pairs (bonding and lone) around a central atom repel each other and arrange themselves to maximize the distance between them.
Key rule: Lone pairs repel more strongly than bonding pairs. This means lone pairs compress bond angles below the ideal values.
The order of repulsion: LP-LP > LP-BP > BP-BP. Each lone pair reduces the bond angle by approximately 2.5°.
"Predict and explain" questions require: (1) State the electron domain geometry, (2) Count lone pairs, (3) State the molecular shape, (4) Give the bond angle.
Common mistake: Confusing electron domain geometry with molecular shape. CH₄ and NH₃ both have tetrahedral electron domains, but NH₃ is trigonal pyramidal because the lone pair is not "seen".
Data Booklet: You must know the shapes and angles — they are NOT given in the exam.