Question

Using VSEPR theory, explain why ethylene is a planar molecule.

Answer 1

ethylene molecule H₂C=CH₂, is linear according to valence shell electron repulsion theory [VSEPR theory].

Atoms with unpaired electrons tend to combine with other atoms which also have unpaired electrons. In this way the unpaired electrons are paired up, and the atoms involved all attain a stable electronic arrangement. This in usually a full shell of electrons (i.e., a noble gas configuration). Two electrons shared between two atoms constitute a bond. The number of bonds formed by an atom is usually the same as the number of unpaired electrons in the ground state, i.e., the lowest energy state. However, in some cases the atom may form more bonds than this. This occurs by excitation of the atom (i.e., providing it with energy) when electrons which were paired in the ground state are unpaired and promoted into suitable empty orbitals. This increases the number of unpaired electrons, and hence the number of bonds which can be formed.

In BF3 the central C atom has the configuration : 1s2, 2s2, 2px 1, 2py 1. During the bond formation one 2s-electron is promoted to vacant 2pz orbital. Thus, excited C (configuration: 1s2, 2s1 , 2Px 1, 2py 1, 2pz 1) has four unpaired eIectrons for bond formation with two Hydrogen atoms and one C atom. The two bonds between C and H atoms should be slightly different strengths, because in one, 2s; while in other, two 2p-orbital electrons are involved, while 2pz1 orbital has one electron and which can overlap with 2pz1 orbital of other C atom sidewise to form pie bond . bond angle of 180°. hence geometry is linear.