Question

What holds combinations of atoms together in molecules?

Answer 1

Hint: the combination of atoms in a molecule is a result of their reactivity. The ability of them to react is due to the electrons present in the outer shell. By reacting them from various types of bonds, like covalent bonds, ionic bonds, metallic bonds, etc. These are either formed by sharing or transfer of electrons.

Complete answer:
The electrons present in the valence or outer shell of any atom makes it reactive. An atom either loses electrons, or gains electrons to achieve a completely filled electronic configuration called a noble gas configuration. Thereby, it forms combinations of atoms called molecules.
When any atom wants to achieve noble gas configuration, it gives rise to bonds, while transferring or sharing of electrons. These are chemical bonds that hold together atoms in a molecule.
There are various bonds which can be formed by atoms. When the electronegativities between the atoms have a wide difference, then they transfer electrons to one another. For instance, when metals react with non – metals, they transfer the 1 or 2 electrons from their valence shell, to achieve fully filled configuration, giving these electrons to non – metals, which have a need of 1 or 2 electrons to completely fill their orbitals. This transfer of electrons results in ionic bonds between atoms.
While, when atoms without any such difference in electronegativities, bond together, they complete their configuration to reach noble gas configuration, by sharing of electrons. For example, carbon has the tendency to share the outer 4 electrons with atoms like hydrogen that are 4 in number to form bonds. These bonds are called covalent bonds, when formed by sharing electron pairs.
Hence, chemical bonds hold a combination of atoms to each other in a molecule.

Note:
Some other bonds are metallic bonds, found in metals, or hydrogen bonds, found in water molecules, or when hydrogen reacts with more electronegative species like Fluorine, oxygen and nitrogen. These bonds give rise to polarity in molecules, creating partial negative and positive charges.