Molecular Geometry

Electron pairs determine molecular geometry

The bonds around an atom, formed through electronic sharing, are made up of pairs of electrons. These pairs of electrons in a molecule occupy defined places in space, just like electrons in atoms. This achieves the minimum repulsion between the electrons. Achieving this minimum repulsion conditions the shape that the molecules adopt.

Balloons tied together at the neck, in groups of two, three and four, that adopt linear, trigonal and tetrahedral shapes

The electron pairs are distributed around the nucleus of the central atom like balloons held together by the inflation nozzle.

BH₃

There are three pairs of bonding electrons around boron. There is nothing else. They are arranged in a flat trigonal shape.

Three balloons in a trigonal planar shape, like the three bonding pairs of BH₃ — **Flat trigonal shape**

CH₄

Lewis structure of methane: carbon shares one electron pair with each hydrogen

There are four pairs of bonding electrons around carbon. There is nothing else. They are arranged in a tetrahedral shape.

Four balloons in a tetrahedral shape, like the four bonding pairs of methane — **Tetrahedral shape**

NH₃

There are four pairs of electrons around nitrogen, but only three are bonding. The arrangement of the electrons is tetrahedral, but the bonds have...

Balloons in a tetrahedral arrangement with three hydrogens: the pyramidal shape of ammonia — **Pyramidal shape**

H₂O

There are four pairs of electrons around oxygen, but only two are forming bonds. The arrangement of the electrons is tetrahedral, but the bonds have...

Balloons in a tetrahedral arrangement with two hydrogens: the angular shape of water — **Angular shape**

In molecular models (JSmol, above), the non-shared electron pairs that nitrogen and oxygen have are "invisible" but they are there implicitly, conditioning the molecular geometry.