Fig 5Shapes of moleculesdrag to turnbond angle 109.5°
lone pairElectron pairs repel each other, so the bonds and lone pairs around an atom spread as far apart as they can. Four pairs point to the corners of a tetrahedron, 109.5° apart. Ammonia and water also have four pairs, but some are lone pairs, which take up more room and squeeze the bonds closer: 107° in ammonia and 104.5° in water. Two pairs make a straight line, three a flat triangle, and six an octahedron.