Double bond is a covalent bond between the two chemical elements which involves four bonding electrons. These bonds have two pairs of shared electrons. It shares two pairs of electrons between two atoms. Normally it is a combination of 2 single bonds. Between two atoms, two short lines are put and this is the representation of double bond.
To understand this, we take an example of oxygen molecule.
Oxygen atom does not contain inert gas and stable electronic arrangement. There is no single atom present in oxygen gas. It contains two atoms and joins forming a molecule.
----- electronic configuration = 2,6
The oxygen’s atomic number is 8 and the electronic configuration is 2, 6. To gain the configuration of inert gas, it requires two more electrons. For the formation of diatomic oxygen molecule, there is a sharing takes place between the two valence electrons of oxygen atom with another oxygen atom. Now each oxygen atom present in covalent molecule has eight electrons in valence shell.
The double bond is stronger than single bond.
In ethane, before the formation of bonds, carbon atoms used to hybridize their outer orbitals, and now they hybridize only three of the orbitals rather than four. They utilize 2s electron and two of the 2p electrons and leave other 2p electron unchanged.
The formation of new orbitals occurs which is known as sp2 hybrids. It is said sp2 because it is made by one s and two p orbitals which reorganize themselves. The arrangement of three sp2 hybrid orbitals is at 120o to each other in a plane. The other p orbital is present at the right angles to them.
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In ethene, two carbon atoms and 4 hydrogen atoms look like this before the joining of both.
Double bonds are shorter than single bonds. The bond order of this is two. They are electron rich which is helpful in making them reactive.