Modern inorganic chemistry

56 STRUCTURE AND BONDING

project to the corners of a tetrahedron. The four valency electrons

of carbon go one into each orbital and overlap of these singly-

occupied orbitals with the four spherical Is orbitals of four hydrogen

atoms gives the tetrahedral methane molecule, with four covalent

bonds.

In ethene the situation is rather different; here, each carbon

atom has one 2s and two 2p orbitals hybridised to form three sp^2

"single-pear' orbitals which are trigonal planar (shown shaded in

each half of Figure 2.10). The remaining 2p orbital is not hybridised,

c c

Figure 2.10. Formation oj the ethene molecule

and remains as a^4 double-pear' (unshaded). The three hybrid
orbitals of each carbon are used thus: two to overlap with the
orbitals of hydrogen atoms to form two C—H covalent bonds, and
one to overlap with the corresponding orbital of the other carbon
atom, along the C.... C axis, giving a C—C bond, as the two halves
of the molecule come together as indicated in Figure 2.10. The
unhybridised 2p orbitals now overlap^4 sideways-on', and we get the
molecule as shown in Figure 2.11.

H

Figure 2.11. The ethene molecule (C—H bonds show as lines for simplicity)

Hence we have two molecular orbitals, one along the line of
centres, the other as two sausage-like clouds, called the n orbital or
n bond (and the two electrons in it, the n electrons). The double
bond is shorter than a single C—C bond because of the 'double'
overlap; but the n electron cloud is easily attacked by other atoms,
hence the reactivity of ethene compared with methane or ethane.