Chemical Bonding: Further Explanation and Representations
Part 2: Covalent Bonding
Covalent bonding results from electrons being shared by atoms rather than being
transferred from one atom to another. Compounds containing only this type of bonding
are called covalent compounds. The driving force behind the formation of a covalent
bond is the tendency for an atom to obtain a stable electron configuration, which is
usually a full valence level. In covalent compounds, a full valence level is achieved by
atoms sharing two or more electrons; the shared electrons are then considered as
simultaneously belonging to the valence levels of both the atoms involved. 1 or more
pairs of electrons move close enough to the nuclei of 2 atoms so that they are attracted by
the nuclei of both. The shared electrons constitute the covalent bond. Covalent bonds are
formed between two nonmetallic elements, that is, between elements whose valence
levels are at least half full.
Usually, but not always, one of the electrons in such a pair comes from the
valence energy level of an atom of one element, and the other electron comes from the
valence energy level of an atom of the other element. The orbital of the one electron
overlaps the orbital of the electron from the other element. Electrons that are positioned
between two atoms in this way are “shared”. When such a bond is formed, the energy of
each of the bonded atoms decreases and the molecule is more stable.
Examples
1. The Hydrogen Molecule
When two hydrogen atoms come close to each other to form the diatomic molecule H2
the hydrogen molecules achieve the same stable configuration as the closest noble gas,
He. We can represent this with an electron dot formula:
H + H  H:H
The shared electrons are simultaneously attracted by the nuclei of both atoms, and this
attraction holds the two atoms together.
When a molecule consists of just two atoms joined together, as in the case of
hydrogen, the molecule can be precisely described as diatomic. When we write the
formula for the hydrogen molecule, we want to show that this element exists as diatomic
molecules rather than as individual atoms. We therefore write H2 instead of H. The
subscript placed to the lower right of the symbol indicates the presence of two hydrogen
atoms in the molecules.
2. The Chlorine Molecule
Like the hydrogen molecule, the chlorine molecule is diatomic. It consists of two
chlorine atoms joined by a covalent bond. Each atom originally has seven electrons in its
valence level and acquires a noble-gas configuration by obtaining a share of the valence
electrons belonging to the other atom:
Cl + Cl  Cl:Cl
We can represent the chlorine molecule as Cl2. Several other nonmetallic elements form
polyatomic molecules. Of these, sulfur and phosphorus are the only non-diatomic
examples.
Note: you are responsible for knowing these polyatomic molecules
Group
H2
VA
N2
P4
VIA VIIA
O2
F2
S8
Cl2
Br2
I2
3. The Hydrogen Chloride Molecule
In the two examples of covalent bonding just discussed, the two atoms sharing electrons
were identical. We will now extend the idea of electron sharing to molecules containing
two or more different atoms.
Hydrogen chloride is a compound formed by hydrogen and chlorine. A hydrogen
atom has just one valence electron. By sharing an additional electron it can acquire the
same electron configuration as helium. Chlorine has seven valence electrons and requires
one more electron to fill its valence level. The requirements of both atoms can be
satisfied if they share the valence electron of hydrogen and one of the valence electrons
of chlorine:
H + Cl  H:Cl
4. The Water Molecule
As you probably know, water is a compound of hydrogen and oxygen. Oxygen, which
has six valence electrons, requires two additional electrons to fill its valence level. This
can be achieved if two hydrogen atoms share their valence electrons with the oxygen
atom. In return, each hydrogen atom shares one of the oxygen atom’s valence electrons:
H + O + H  H:O:H
Note: If only one atom of an element is present in the molecule, no subscript is used after
the symbol of that element in the chemical formula. Thus we represent the water
molecule by H2O rather than by H2O1. The formula of a covalent compound tells us
exactly how many atoms of each element are contained in one molecule of that
compound.
5. The Carbon Dioxide Molecule
The covalent bonds we have discussed so far have all been formed by atoms
sharing one pair of electrons. A bond that consists of just one shared pair of electrons is
known as a single bond. Many molecules, however, have covalent bonds that consist of
two or three electron pairs shared by two atoms. If two pairs of electrons (i.e. a total of
four electrons) are shared by two atoms, the bond formed is called a double bond.
An example of a molecule containing double bonds is carbon dioxide. The
carbon dioxide molecule consists of a central carbon atom and two oxygen atoms.
Carbon has four valence electrons and hence needs four more to attain a noble-gas
configuration. Oxygen has six valence electrons and therefore needs two more to fill its
valence level. For all the atoms to be satisfied, each oxygen atom must share two of its
electrons with the carbon atom. At the same time, carbon must share two of its electrons
with each oxygen. Thus we have
O + C + O  O::C::O
The compound formed is called carbon dioxide. The prefix di in the word “dioxide”
indicates the presence of two oxygen atoms.
6. The Nitrogen Molecule
When six electrons are shared by two atoms, a triple bond is formed. The diatomic
nitrogen molecule, the major component of air, is an example of a molecule with a triple
bond. A nitrogen atom has five valence electrons, which is three electrons short of a
noble-gas configuration.
N + N  N:N  N::N  :N:::N:
Coordinate Covalent Bonds
Although equal sharing of electrons can be used to explain the bonding in most simple
covalent compounds, the concept cannot be successfully applied to all cases. In 1923
Nevil Sidgwick, a British chemist, showed that covalent bond formation could involve
the sharing of an electron pair donated by only one of the two atoms. This situation
occurs in the formation of the ammonium ion (NH4+1) from ammonia (NH3) and the H+1
ion:
In the ammonia molecule, the nitrogen has three bonding electrons plus one lone
pair of electrons that is not involved in bonding. The H+1 ion, which is the nucleus of a
hydrogen atom, has no electrons. It has a positive charge and is attracted by the lone
pair of electrons of the nitrogen atom. The resulting covalent bond is, in all respects,
equivalent to the other three covalent bonds between the nitrogen and hydrogen atoms.
In this case, however, both of the electrons involved in bond formation come from the
nitrogen atom. Sidgwick called this type of a bond a coordinate covalent bond.
In the ammonium ion, the positive charge is spread out over the entire ion. This is
represented by writing the electron-dot formula inside square brackets with the net ionic
charge shown as a subscript.
Important Points about Covalent Compounds:
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-
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Covalent compounds result from sharing electrons between atoms to form
covalent bonds. Covalent bonds are formed between atoms that have
similar properties, or have similar electronegativities.
Compounds formed from atoms that tend to share electron pairs equally
with each other are said to be nonpolar covalent compounds.
Compounds formed from atoms that share unequally (electronegativities are
different) Are called polar covalent compounds. The electron pairs are
pulled toward the atom that has the higher electronegativity.
Polar covalent compounds are “slightly” positive at one end, and “slightly”
negative at another. We show these “Slight” charges with a delta, δ,
symbol
o Example:
 When a bond is formed between hydrogen and oxygen, or
between carbon and oxygen, the electrons in the bond are
pulled toward the oxygen.
Questions:
1. How many electrons are shared in forming each of the following:
a. A single bond
b. A double bond
c. A triple bond.
2. Write the electron-dot formula for each of the following
compounds
d. SiH4
a. OF2
b. ClF
e. CS2
c. H2S
f. H2O
3. Identify which of the following molecules have polar covalent
bonds of these indicate which of the atoms are slightly positive and
which are slightly negative. Hint use your electronegativity tables!
i. Cl2
ii. NH3
iii. CO
iv. H2O
v. OF2