A double bond is a type of covalent bond formed when a non-metallic atom shares a pair of electrons with another non-metallic atom that in turn shares another two electrons. A total of four electrons get shared between the atoms. A chemical bond happens because atoms like to have their valence shells fully occupied with electrons, to satisfy the octet rule. And the atoms are able to stick together due to the electrostatic force existing between the proton(s) in the nucleus of one atom and electron(s) of the other atom.
In case of disparity, atoms try to alter the imbalance by either pulling atoms with sufficient electrons toward them or getting attracted to another atom. Oxygen, carbon dioxide, ethane, etc. are molecules with double bonds. This bond may exist between atoms of the same element or different elements. In a structural chemical formula, a double bond is represented by two parallel lines.
A double bond comprises a sigma bond, which is a head-one overlap between the bonded atoms; and a pi bond, which is a side-on overlap between the atoms. A double bond is stronger than a single bond but is weaker than a triple bond, which also has its share of sigma and pi bonds. A single bond has just one sigma bond but no pi bonds. Compared to pi bond, sigma bond is stronger. A double bond has both sigma and pi bond, which is one of the reasons why a double bond is stronger than a single bond.
The increased bond strength means shorter length. This means a double bond is shorter in length than a single bond but longer than a triple bond. Breaking this dual bond needs three times the energy that’s required to separate atoms attached to each other in a single bond.