Covalent Bonding, The Octet Rule and Multiple Bonds
Ionic substances exist as a metal cation and a nonmetal anion that are attracted to one another. Covalent
molecules occur when non-metals combine with nonmetals and electrons are shared in order to lower
energy. The process of forming a covalent bond can be
summarized as follows:
1) two atoms approach one another as each has a high
electron affinity
2) the electron density in each atom begins to shift
towards the nuclei of the other atom
3) there is now a much greater possibility of finding an
electron in the region between the 2 atoms
4) each of the atoms appears to have a share of the two
electrons between them
5) a balance between the two electron repulsions and
the two nuclear repulsions occurs. This creates a
lower energy situation in a delicate balance between
repulsion and attraction
Every covalent bond has 2 important characterisics:
a) the average distance between the nuclei (bond
length)
b) the energy required to separate the 2 nuclei and
create 2 neutral atoms (bond energy)
NOTE: when a bond is formed, energy is released.
When a bond is broken, energy is required. This
will be discussed in the thermochemistry section.
Covalent bonds can be shown as 2 dots between
atoms or as a dash (we will use the former)
Example: H + H  H : H
The Octet Rule
We know from electron configurations that noble
gases are very stable and low energy. When ions form,
electrons tend to be gained or lost until a noble gas
configuration is reached. The same is true for the
number of electrons shared in covalent molecules. The
octet rule states that when atoms react, they tend to
achieve a valence shell having eight electrons.
Draw Lewis Dot Diagrams for the following:
1) methane (CH4)
2) ammonia (NH3)
3) water (H2O)
Multiple Bonds
Bonding that we have encountered so far only have
single bonds. There are however many covalent
molecules in which more than one pair of electrons are
shared between 2 atoms. Such an example would be
nitrogen gas, N2.
CO2 works in the same fashion:
Homework:
Draw Lewis dot diagrams for the following:
a) LiBr b) K2Sc) PH3 d) F2 e) Na3N f)
Al2S3
g) CaS
h) BaO i) O2 j) SiO2 k) CS2 l)
BF3