Covalent bond. Image credit: Flickr
Covalent bond. Image credit: Flickr

Also called a molecular bond, a covalent bond is a type of chemical bond wherein two non-metallic atoms share (not donate) their electron(s) with each other and stick together, as a result. This attraction is due to the electrostatic force existing the electron(s) of one atom and proton(s) in the nucleus of the other atom. As a result, the electrons become common to both the atoms. Neither atom in a covalent bond can claim exclusive ownership of the shared electron(s).

The bonded atoms do not always share their electrons equally. There are instances when a particular atom in a covalent bond could share one electron and the other may share more than one. This is usually the case when atoms from two different elements bond. If the bonding atoms belong to the same element, then the sharing is equal.

The concept of electron-sharing between atoms was first brought to light in 1916 by G.N. Lewis, an American chemist. The electrons that get shared are referred to as bond pair, and the ones not part of the picture are called lone pair. For example, the water molecule (H2O) has two hydrogen atoms – the two atoms have a covalent bond going on between them.

Covalent Bond Types

There are different types of covalent bonds.

  • Single bond: A single bond entails two valence electrons, or the atoms share an electron each.
  • Double bond: A double bond is when four valence electrons get shared – two electrons shared by each atom.
  • Triple bond: A triple bond involves sharing of six electrons – three electrons shared by each atom.
  • Polar covalent bond: This covalent bond type entails electrons that are not shared equally. The atom with higher electronegativity (tendency to attract electrons) ends up sharing lesser electrons.
  • Non-polar covalent bond: The electronegativity of both the atoms are the same, which means electrons are shared equally.

Bonding for Stability

Atoms usually bond to stabilize themselves and adhere to the octet rule. When the outer ring or the valence shell isn’t filled with electrons to capacity, the atom tends to fill up the empty space or get rid of the electrons sticking out as a sore thumb. Depending on the number of electrons in the outer shell, an atom may choose to lose or gain electrons.

If there are few electrons in the shell, the atom would be happy to give it away to another element’s atom. If it’s the other way around, the atom would be keen to borrow electrons from another atom. When an atom loses or borrows an electron, it ends up getting physically attached to the other atom. Covalent bonds may include sharing a single electron or more.

Bond Strength

Most molecules are made through covalent bonds. A covalent bond is generally strong and stable. The strength of a covalent bond depends on the number of electrons involved in the bonding. The bond strength also depends on the distance between the nuclei of the two atoms.

The farther the distance, the weaker is the bonding; and the bond gets stronger if the nuclei come closer. Moreover, with increasing bond strength, the bond length reduces. This is why a triple bond is stronger than a double bond (involving atoms from the same element), but it’s shorter in length than a double bond.

Covalent bond strength can be determined by the energy required to separate the attached atoms. With increasing bond strength, the energy needed to break the bond would also go up. The energy that helps break a covalent bond is called bond dissociation energy or bond energy.