THE SHAPE OF MOLECULES

1. Molecular shape

The individual molecules of a compound are solid objects, regardless of the physical state of that compound. This means that like all solid objects, molecules have definite shapes. The simplest ones are:

2. Valence shell electron pair repulsion (VSEPR) theory

The VSEPR theory () enables one to predict the shape of molecules by considering the covalent bonds and non-bonding pairs of electrons of the central atom, that is, the atom to which other atoms are attached.

Remember that a single covalent bond consists of a molecular orbital containing two electrons of opposite spins, one provided by each atom that participates in the bonding. These orbitals will be regions in space where there is a predominantly negative charge, caused by the electrons that are accommodated in the orbital. Consequently, orbitals will tend to repel one another.

The theory requires that the molecular orbitals dispose themselves in such a way as to minimise the electrostatic repulsion forces between them, thereby reducing the energy of the molecule. Lone pairs of electrons, that is, electron pairs that are not linking two atoms together, also form molecular orbitals and therefore will be involved in electrostatic repulsion with bonding molecular orbitals. Basically, the sum of the electrostatic repulsions, and thus the energy, will be a minimum when the orbitals are equally separated.

Methane

Methane has a central atom, carbon, linked covalently to four hydrogen atoms. In order to minimise the electrostatic interaction between the four C-H bonds (the diagram above shows the 6 different interactions that are possible), which occurs when the four hydrogen atoms are equally separated, the carbon atom must lie at the center of a regular tetrahedron, with the four H-C-H angles each equal to 109.5. Hence methane is a tetrahedral molecule.

Ammonia

Here, the central atom is nitrogen, attached to 3 hydrogen atoms. Bear in mind that there is a non-bonding pair of electrons residing on the nitrogen atom. This pair of electrons is also involved in repulsive interactions, and so must be considered. It turns out that non-bonding electron pairs exert a greater electrostatic repelling force than bonding electron pairs, and this causes the three H-N-C bonds to be "squeezed" together, resulting in a smaller bond angle (106) than that expected for a regular tetrahedron (109.5).

Note that while the three N-H bonds and the non-bonding orbital are directed towards the corners of a tetrahedron, the four atoms lie at the corners of a three-sided pyramid, hence the overall shape of the molecule is pyramidal.


Water

The central atom is oxygen, attached to two hydrogen atoms. The oxygen atom, after sharing two electrons with each hydrogen atom, is left with four electrons not taking part in bonding, and these form two non-bonding pairs of electrons. Their combined repulsive influences on the two H-O bonds result in a smaller bond angle (104.5) than expected for a regular tetrahedron.

Since there are only three atoms, they must necessarily lie in one plane, and so water is a planar molecule.


Carbon dioxide

The central atom is carbon, bound to oxygen by two C=O double bonds. Each oxygen atom has two non-bonding pairs. The arrangement of least energy is found when all three atoms lie in a straight line, so carbon dioxide is a linear molecule.


Boron trifluoride:

Boron trifluoride, BF3, has three fluorine atoms attached to a central boron atoms. There are no non-bonding electron pairs (compare this with ammonia), and thus the minimum energy requirements will be met when the four atoms lie in one plane, with the three F-B-F bonds all being exactly 120. Note that the central boron atom does not have an octet of eight electrons - it has only six. For this reason, boron trifluoride is known as an ELECTRON DEFICIENT molecule.


3. Additional questions