How do I predict the shape and bond angle of an molecule?
Electron pairs arrange themselves so they are as far apart as possible (electron pair repulsion theory). In a covalent compound, the number of electron pairs around the central atom determines the shape of the molecule.
An electron pair shared between two atoms is called a bonding pair (BP)
An electron pair on one atom only- not shared- is called a lone pair (LP)
BeCl2 has two bonding electron pairs shared between the Be2+ ion and the two Cl- ions. Cl-Be-Cl bond angle = 180 degrees (linear)
BF3 has three bonding electron pairs shared between the B3+ ion and the three F- ions. The bond angle = 120 degrees (trigonal)
CH4 has four bonding electron pairs shared between the C4- ion and the four H+ ions. The bond angle = 109.5 degrees (tetrahedral)
However when we introduce lone pairs to the molecules it distorts the bond angle because of electron pair repulsion.
Electron pair repulsion theory tell us that LP-LP repulsion > LP-BP repulsion > BP-BP repulsion.
Therefore a molecule containind three bonding pairs and one lone pair like NH3 (four electron pairs all together) gives rise to a distorted bond angle because the lone pair repels the other bonding pairs which decreases the bond angle from 109.5 (tetrahedral) to 107 (pyrimidal).
