ATOMIC STRUCTURE OF CARBON

Atomic orbitals

The atomic orbitals available for the six electrons of carbon are the s orbital
in the first shell, the s orbital in the second shell and the three p orbitals in
the second shell. The 1s and 2s orbitals are spherical in shape. The 2p
orbitals are dumbbell in shape and can be assigned 2px, 2py or 2pz depending
on the axis along which they are aligned.

Energy levels

The 1s orbital has a lower energy than the 2s orbital which has a lower
energy than the 2p orbitals. The 2p orbitals have equal energy (i.e. they
are degenerate).

Electronic configuration

Carbon is in the second row of the periodic table and has six electrons which
will fill up lower energy atomic orbitals before entering higher energy
orbitals (aufbau principle). Each orbital is allowed a maximum of two electrons
of opposite spin (Pauli exclusion principle). When orbitals of equal
energy are available, electrons will occupy separate orbitals before pairing
up (Hund’s rule). Thus, the electronic configuration of a carbon atom is 1s2
2s2 2px
1 2py
1.

Covalent bonding

A covalent bond binds two atoms together in a molecular structure and is formed
when atomic orbitals overlap to produce a molecular orbital – so called because
the orbital belongs to the molecule as a whole rather than to one specific atom. A
simple example is the formation of a hydrogen molecule (H2) from two hydrogen
atoms. Each hydrogen atom has a half-filled 1s atomic orbital and when the atoms
approach each other, the atomic orbitals interact to produce two MOs (the number
of resulting MOs must equal the number of original atomic orbitals, Fig. 1).

The MOs are of different energies. One is more stable than the original atomic
orbitals and is called the bonding MO. The other is less stable and is called the
antibonding MO. The bonding MO is shaped like a rugby ball and results from
the combination of the 1s atomic orbitals. Since this is the more stable MO, the
valence electrons (one from each hydrogen) enter this orbital and pair up. The
antibonding MO is of higher energy and consists of two deformed spheres. This
remains empty. Since the electrons end up in a bonding MO which is more stable
than the original atomic orbitals, energy is released and bond formation is
favored. In the subsequent discussions, we shall concentrate solely on the bonding
MOs to describe bonding and molecular shape, but it is important to realize
that antibonding molecular orbitals also exist.

adapted from G. L. Patrick
Department of Chemistry and Chemical Engineering,
Paisley University, Paisley, Scotland