covalent bond
Imagine two children who each want a toy but can only afford one between them, so they agree to share it and play together. Two atoms can do the same with electrons: instead of one giving them away, both contribute and the electrons belong to both at once. That shared electron arrangement is a covalent bond.
Precisely, a covalent bond is a chemical bond in which two atoms share one or more pairs of electrons. The shared pair sits in the region between the two nuclei, and both positive nuclei are attracted to it, which pulls the atoms together. Sharing one pair makes a single bond, two pairs a double bond, and three pairs a triple bond — more shared pairs generally mean a shorter, stronger bond.
Covalent bonding is the rule for nonmetals bonding with nonmetals, and it is what builds molecules — water, oxygen gas, sugar, DNA, plastics. A caveat: sharing is rarely perfectly even. When the two atoms differ in how strongly they pull on electrons, the shared pair sits closer to one of them, making the bond polar rather than purely covalent.
Two hydrogen atoms each bring one electron and share them, forming H₂ with a single covalent bond. The shared pair is attracted to both nuclei at once, which is why the two atoms stay together rather than drifting apart — it costs energy to pull them back into separate atoms.
A single covalent bond is one electron pair shared between two atoms.
Sharing electrons is not the same as them sitting still in the middle. The shared pair is constantly in motion, spread out over a region between the nuclei; the bond is the net attraction this cloud creates, not a static stick joining two balls.