A covalent bond forms when atoms share electron density in a stable molecular arrangement.

A simple bond is often represented by a line:

H—H

The line is a compact chemical notation for a shared bonding interaction, not a literal rod between nuclei.

Bond polarity

Different elements attract shared electron density with different strengths. Electronegativity is a relative measure of this tendency.

When two bonded atoms have similar electronegativities, the bond is approximately nonpolar. When one atom attracts the shared electrons more strongly, the bond becomes polar and carries partial charges:

$$\delta^+!A-B!\delta^-.$$

Multiple bonds

Two atoms can share more than one pair of electrons, producing double or triple bonds. Bond order affects bond length, strength and molecular geometry.

Bond energy

Breaking a stable covalent bond requires energy, while forming the bond releases energy relative to the separated arrangement. Chemical reactions rearrange bonds and are governed by the energy and entropy of the complete system.

Covalent bonds determine which atoms form one molecule; weaker interactions between molecules are described separately.