Covalent Compounds

Return to the Atomic Structure and Bonding Page.
<= Back Next =>
Most substances that we see are not ionic. They are not collections of electrically charged particles (ions), but are electrically neutral combinations called molecules.
In ionic bonding, the potential energy lowering must occur. But many times, it is not possible because of the great range of IE.
For example, lets take a hydrogen molecule (H2). As two H atoms approach each other, the electron of each atom feels the pull from the other atom. The electron density around each nucleus shifts to include both atoms. To lower the energy needed, the two nuclei share the electrons. Since the two electrons and the two nuclei repel each other but the electrons and nuclei attract each other, the two nuclei are kept a certain distance away from each other, called a bond length or bond distance. when the bond is formed, an amount of energy is release as the potential energies are lowered. This energy is called the bond energy.

Formation of a Hydrogen Molecule

Before they join, the 1s orbitals of each H atom is only half-filled. When they join, they share each other's electrons, filling each other's 1s orbital pair.
More simply, an electron pair can be represented by a dash. 
H-H
Formulas drawn with Lewis symbols are called Lewis formulas or Lewis structures, or more simply structural formulas. They show which atoms are present and how they are connected into each other.

Covalent Bonding and the Octet Rule

Applied to covalent bonding, the octet rules states that when atoms form covalent bonds, they tend to share sufficient electrons so as to achieve an outer shell having eight electrons (except for hydrogen and helium, which have a stable outer shell of 2 electrons). For example, here is the bonding of hydrogen Chloride (HCl):
An octet is formed on the chlorine atom and the maximum pair on the hydrogen atom. Here is the bonding of a chlorine molecule (Cl2):
An octet is formed on the two chlorine atoms.
Some atoms can also form more than one covalent bond. For example, carbon can form 4 bonds (because it has 4 valence electrons), nitrogen can form 3 (5 valence electrons), and oxygen can from 2 (6 valence electrons):

Double and Triple Bonds

So far, the bonds we've studied were single bondS, where only one pair of electrons were shared between atoms. To illustrate a triple bond, the nitrogen gas molecule will be used. A nitrogen atom looks like this, with 5 valence electrons around it.
When the two N atoms share electrons they have to share 3 pairs to add to their 5 electrons to complete their octet.
Double bonds occur in carbon dioxide (CO2):

Coordinate Covalent Bonds

For example, let's take ammonia (NH3), which has one unshared pair.
If it is placed in an acidic solution (a proton donator), H+ ions (protons), the ammonia takes in the proton and becomes NH4+, an ammonium ion. The H+ ion has a vacant 1s orbital in its valence shell and can accomodate two electrons. The two unshared pair of electrons in the ammonia can be donated to the H+.
An arrow from the nitrogen atom to the hydrogen ion indicates that the nitrogen is sharing it pair of electrons with it. This sharing is called an coordinate covalent bond.
Next =>