[[1]] holds combinations of atoms together in molecules.

The reactivity of the atoms in a molecule determines how they are arranged. The electrons in the outer shell provide them the capacity to reacted. by forming them from several sorts of bonds, such as covalent, ionic, metallic, etc. These are either created by electron sharing or transfer.

Any atom is reactive because of the electrons in its valence, or outer, shell. To acquire the electrical configuration known as a noble gas configuration, an atom either adds electrons or loses electrons. By doing so, it creates atom-atom pairings known as molecules.

Any atom that desires to be in the configuration of a noble gas forms bonds while transferring or sharing electrons. Atoms in a molecule are held together by these chemical bonds.
Atoms are capable of creating a wide range of bonds. Atoms transmit electrons to one another when there is a significant variation in the electronegativities between them. To reach a fully filled configuration, for instance, metals react with non-metals by transferring one or two electrons from their valence shell to the non-metals, who require one or two electrons to fully fill their orbitals. Ionic bonds form between atoms as a result of this electron transfer.

While atoms that join together without any such variation in electronegativities complete their configuration to become noble gases by sharing electrons. For instance, while forming bonds, carbon has a propensity to share its outer four electrons with other four-atom atoms, such as hydrogen. When these connections are created by sharing electron pairs, they are referred to as covalent bonds.
Thus, in a molecule, a group of atoms are held together by chemical bonds.

Hence, the answer is Chemical bond.