What allows two or more atoms to be held together?

Atoms come together to form molecules because of their electrons. Electrons can join (or bond) atoms together in two main ways. When two atoms share electrons between them, they are locked together (bonded) by that sharing. These are called covalent bonds.

Covalent Bonds

Explanation:

The nature of the covalent bond is determined by the number of electrons shared and the nature of the two elements sharing the bond. Two or more atoms held together by covalent bonds in a specified arrangement is called a molecule.