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Covalent Bonds
Covalent chemical bonds involve the sharing of a pair of
valence electrons by two atoms, in contrast to the
transfer of electrons in ionic bonds. Such bonds lead to
stable molecules if they share electrons in such a way as
to create a noble gas configuration for each atom.
Hydrogen gas forms the simplest covalent bond in the
diatomic hydrogen molecule. The halogens such as
chlorine also exist as diatomic gases by forming covalent
bonds. The nitrogen and oxygen which makes up the
bulk of the atmosphere also exhibits covalent bonding in
forming diatomic molecules.
Covalent bonding can be
visualized with the aid of
Lewis diagrams
+ O Ca^
Ca O
Na + Cl Na
+ Cl
Sodium contributes electron, leaving it with a closed shell
Chlorine gains electron, leaving it with a closed shell
DOUBLE AND TRIPLE BONDS:
Try to draw the Lewis Dot symbol for carbon dioxide, CO 2. You will encounter a problem. Carbon has 4 valence electrons, and the two oxygen together have 12 ,( 2 x 6 ), for a total of 16 electrons. The two possible structures that can be drawn for carbon dioxide can be these:
In either case, carbon or oxygen will not have eight electrons represented. However, if each oxygen atoms shares two pairs of electrons with the carbon atom, double bonds would be formed, and octets around both carbon and oxygen can be achieved.
The circles are drawn to represent the octet for each atom.
A Double Bond is a covalent bond in which four electrons (two pairs) are shared by the bonding atoms.
Triple Bond is a covalent bond in which two atoms share three pairs of electrons.
Nitrogen gas is an example of a triple bond.
between the possible symbol.
Electronegativity
Electronegativity is a measure of the ability of an atom in a molecule to draw bonding electrons to itself.
The most commonly used scale of electronegativity is that developed by Linus Pauling in which the value 4.0 is assigned to fluorine, the most electronegative element.
An important application of electronegativity is in the prediction of the polarity of a chemical bond.
Lewis Dot Diagrams of Selected Elements
