Covalent bondBond may be of different types.
Electrons in a covalent bondIn the formation of molecule only the electrons present in outermost shell or valence shell of atoms normally participate while the electron belongs to inner energy shell are mostly not involved in the bond formation.
Remember: A bond will form only when combining atoms have minimum energy and maximum stability.This will possible if the atom complete the octet (8 electrons in outermost shell) and achieve the electronic configuration of nearest noble gas atom.
In covalent bond the combining atoms mutually contribute one, two or three electrons depending upon their requirement the bond formed.This leads the formation of single double and triple bond.
Covalent nond is generally present in the atoms of non metal. This may be either same or different. In case of atoms are same the covalent molecule is known as homoatomic and if the combining atoms are different then it is called as heteroatomic molecules
Homoatomic molecules(non polar covalent): The simplest among them is hydrogen H2 molecule in which both the participating atoms have one electron each initially, share there electron and both achieve the stable electronic configuration.
Another example:Chlorine molecule Cl2 .In this case both the chlorine atoms that is equals to (atomic no 17) have 7 valance electron and a short of one electron each.They share one electron pair which is an electron is contributed by both the atoms to achieve nearest noble gas configuration.
The electron pair belonging to individual atoms not involving sharing called as lone pair (example:oxygen molecule).
Homoatomic molecules(polar covalent): In this case the atom taking part in bond formation belongs to different elements. A few examples of heteroatomic molecules are:
It should be noted that this the structure of molecules or ions in which shared electrons are represented by the dots are known as Lewis dot structure. While those in which these are indicated by line or dashed are called as Couper structures.