Unstable atoms tend to bond with other atoms by losing, gaining or just sharing their electrons in order to attain complete electrons in their outermost shell, which is more stable. Covalent bond only occurs when electrons are being shared between atoms.

What is Covalent Bonding?

Covalent bonding is the sharing of electrons between atoms of the same or different elements.

In the periodic table, covalent bonding only occurs among atoms of group 5, 6, 7 which are all non-metal elements. Also, the group 4 elements which are metalloids often participate in covalent bonding than ionic bonding.

In covalent, each atom that is taking part will contribute the number of electrons equal to its number of covalency for sharing.

Covalency of an atom

is the remaining number of electrons required to complete its outermost shell.

In Oxygen bonding.

An oxygen atom has 6 electrons in its outermost shell, and so, needs more 2 electrons, making a total of 8, to complete its outermost shell. Therefore, it's covalency is 2, and in covalent bonding, its atom will only share 2 electrons. And these 2 electrons it is going to share will come from the 6 electrons it already has. You can observe the oxygen atoms in O₂, H₂O, H₂O₂.

In Nitrogen bonding.

A nitrogen atom has 5 valence electrons, and so, needs more 3 electrons making a total of 8, to complete its valence shell. Therefore, its covalency is 3, and will only share 3 electrons during covalent bonding. These 3 electrons it is going to share will be taken from the 5 electrons it already has in its valence shell. Example in NH₃, NO₂.

In Hydrogen bonding.

A hydrogen atom has one electron in its valence shell, and needs one more electron, making a total of 2, to complete the shell. Therefore, its covalency is 1, hence will only share one electron. The one electron to be shared will be that only one electron it already has in its shell. Examples in H₂, CH₄, H₂O₂, H₂O.

In Chlorine bonding.

A chlorine atom has 7 valence electrons, and needs one more electrons, making a total of 8, to complete the valence shell. Therefore, its covalency is 1, hence will only share one electron. Examples in Cl₂, PCl₃.

In Carbon bonding.

A carbon atom has 4 valence electrons, and requires an addition of 4 electrons to complete its valence shell. Therefore, its covalency is 4, hence will only share 4 electrons during covalent bonding. Examples in CH₄, CO₂.

Hints in Covalent Bonding

1. In covalent, it's also good to know that atoms do not lose those electrons when shared. There's a lot different between LOSING and SHARING!

2. In covalent bonding, the participating atoms each contribute their electrons for them to share. The both atoms most contribute!

3. The shared electrons in a covalent bond/molecule do not belong to a particular atom, rather they belong to both atoms, so that the both atoms can attain a stable duplet or octet structure.