Covalent Bonding

Basic Concepts of Chemical Bonding

Covalent Bonding

Covalent Bonding

Ionic substances:

usually brittle
high melting point
organized into an ordered lattice of atoms, which can be cleaved along a smooth line

the electrostatic forces organize the ions of ionic substances into a rigid, organized three-dimensional arrangement

The vast majority of chemical substances are not ionic in nature

gases and liquids, in addition to solids
low melting temperatures

G.N. Lewis

reasoned that an atom might attain a noble gas electron configuration by sharing electrons

A chemical bond formed by sharing a pair of electrons is called a covalent bond

The diatomic hydrogen molecule (H₂) is the simplest model of a covalent bond, and is represented in Lewis structures as:

The shared pair of electrons provides each hydrogen atom with two electrons in its valence shell (the 1s) orbital.

In a sense, it has the electron configuration of the noble gas helium

When two chlorine atoms covalently bond to form Cl₂, the following sharing of electrons occurs:

Each chlorine atom shared the bonding pair of electrons and achieves the electron configuration of the noble gas argon.

In Lewis structures the bonding pair of electrons is usually displayed as a line, and the unshared electrons as dots:

The shared electrons are not located in a fixed position between the nuclei. In the case of the H₂ compound, the electron density is concentrated between the two nuclei:

The two atoms are bound into the H₂ molecule mainly due to the attraction of the positively charged nuclei for the negatively charged electron cloud located between them

For the nonmetals (and the 's' block metals) the number of valence electrons is equal to the group number:

Element	Group	Valence electrons	Bonds needed to form valence octet
F	7A	7	1
O	6A	6	2
N	5A	5	3
C	4A	4	4

Examples of hydride compounds of the above elements (covalent bonds with hydrogen:

Thus, the Lewis bonds successfully describe the covalent interactions between various nonmetal elements

Multiple bonds

The sharing of a pair of electrons represents a single covalent bond, usually just referred to as a single bond

In many molecules atoms attain complete octets by sharing more than one pair of electrons between them.

Two electron pairs shared a double bond

Three electron pairs shared a triple bond

Because each nitrogen contains 5 valence electrons, they need to share 3 pairs to each achieve a valence octet.

N₂ is fairly inert, due to the strong triple bond between the two nitrogens
The N - N bond distance in N₂ is 1.10 Å (fairly short)

From a study of various Nitrogen containing compounds bond distance as a function of bond type can be summarized as follows:

N-N 1.47Å
N=N 1.24Å
N=N 1.10Å

As a general rule, the distance between bonded atoms decreases as the number of shared electron pairs increases

1996 Michael Blaber