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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)

BeClhas two bonding electron pairs shared between the Be2+ ion and the two Clions. Cl-Be-Cl  bond angle = 180 degrees (linear)

BF has three bonding electron pairs shared between the B3ion and the three Fions. 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 NH(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).

Tolu T. GCSE Biology tutor, GCSE Chemistry tutor, GCSE Physics tutor,...

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