It is formed by the sharing of electron pair between bonded atoms.
The atom with larger electronegativity attract the electron pair more towards it self and becomes partial negative while the other atom becomes partial positive.
For example:
In water the electronegativity of oxygen is 3.44 and hydrogen is 2.2. That's why electron pair attracted more towards oxygen, thus oxygen becomes partial negative and hydrogen becomes partial positive. Oxygen attract two electron, one by each H atom and thus complete the octet.
O₈ = 1s² 2s² 2p⁴
To complete the octet oxygen needs two electron thus it gets the electronic configuration of Neon.
While hydrogen atom need one more electron to complete the octet and gets electronic configuration of noble gas element He.
