Question

How are atoms and molecules different?

Answer 1

The main difference relies on the fact that molecules are sets of atoms.

Explanation:

Atoms are the smallest part of a chemical element which can be obtained. There are smaller things than atoms (for instance, particles or quarks), but they do not represent any kind of element (in the same way that screws and gears are not real engines, but a part of them).
An atom is composed by a nucleus, which includes protons and neutrons, and an electron shell. Interactions between electrons from different atoms (by exchanges or sharing, for example) give place to chemical bonds. An union between atoms is called molecule.

However, not every atomic union is called molecule. For instance, in metals, atoms are united ones to others; however they do not form molecules, but a lattice.
So, what is the difference?

Molecular compounds are composed exclusively by molecules. For instance, water is composed by `"H"_2 "O"` molecules. 200 molecules of `"H"_2 "O"` are water; 400 atoms of H and 200 atoms of O, without any bond between them, are not water.
Nevertheless, 200 atoms of Fe are iron, and 10 atoms do too.

In addition, molecules interact among them, by forces called Van der Waals forces.