Covalent bonding occurs when electrons are shared between atoms. Double and triple covalent bonds occur when four or six electrons are shared. Double and triple bonds are indicated by a double or triple line connecting one atom to another. It is of importance to note that only atoms with the ability to gain or lose at least two valence electrons can participate in double bonds and those can gain or lose at least three valence electrons can participate in triple bonds.
Double and triple bonds can be explained by orbital hybridization, the mixing of atomic orbitals to form new hybrid orbitals. Hybridization describes the bonding atoms from a specific atom's point of view. When one or multiple electrons become excited, they move to a higher level of energy. This promotion of electrons effectively changes the nuclear charge of the atom. The solution of this configuration is a linear combination of s and p wave functions, which equate to hybridized orbitals. The newly hybridized orbitals are of the same energy level and are denoted as spx, where s and p denote the orbitals and the value of x ranges from 1-3, depending on how many p orbitals are available for bonding.
The simplest example of a compound with a double bond is ethylene (or ethene), C2H4.
From the carbon atoms' perspective, each has three sp2 hybrid orbitals and one p orbital; the sp orbitals arise from each carbon "hybridizing" with the other carbon as well as with two hydrogen atoms. But carbon has two p orbitals; since each carbon's last p orbital is unused, it remains a p orbital. The three sp2 orbitals lie in a single plane at 120-degree angles. As the carbon atoms approach each other, their orbitals overlap and form a bond. Simultaneously, the p orbitals approach each other and form a bond. To maintain this bond, the p orbitals must stay parallel to each other; therefore, rotation is not possible. bonds are not as strong as bonds because of smaller overlap between the p orbitals, and they are shorter because the p orbital overlap is maximized; however, double bonds together are stronger than a single bond. A double bond restricts the structure of the compound; the double-bonded atoms and all atoms bonded to them lie in a flat plane. Because the bond must stay parallel, there is no rotation around the double bond.
A triple bond involves the sharing of six electrons, in a sigma bond and two bonds.The simplest triple-bonded compound is acetylene, C2H2. Triple bonds are stronger than double bonds due to the the presence of two bonds rather than one. Each carbon has an sp orbital, and they overlap to form an sp-sp bond. The remaining four p orbitals approach each other and form two bonds, resulting in one and two bonds. Similar to double bonds, no rotation around the bond axis is possible.