Final answer:
Atoms are held together by the electromagnetic force between oppositely charged particles, while the strong nuclear force keeps the protons and neutrons together within an atom's nucleus.
Step-by-step explanation:
The force that holds atoms together is primarily the electromagnetic force, which is responsible for the attractive force between oppositely charged particles (electrons and protons) and the repulsion between particles of the same charge. In the case of molecules like water, a chemical bond is the force that holds the atoms together, which involves atoms sharing electrons, such as with a covalent bond. Within the atom's nucleus, the strong nuclear force overcomes the electrostatic repulsion between positively charged protons, holding the nucleus together. This force, which acts between protons, neutrons, and between protons and neutrons, is very strong at very small distances but quickly diminishes beyond the scale of a nucleus.