posted by PInk Floyd .
Why is H2O a bent molecule, while BeH2 is linear?
Why is BF3 a planar molecule, while NH3 is not?
Be is in group II and it has two electrons; therefore, addition of two H atoms makes H:Be:H (although Be doesn't have an octet in this structure) so it has two regions of high electron density (sp hybridization) and that is a linear molecule. Oxygen is in group VI, it has six electrons, H2O has 4 regions of high electron density which makes the electronic geometry tetrahedral (sp3 hybridization). The two sets of unshared electrons makes it a distorted tetrahedron so the molecular geometry is bent.