Summary of "Difference between an Atom, a Molecule and a Compound"

Difference between an atom, a molecule, and a compound

The smallest unit of matter.
Composed of subatomic particles:
- Protons — positively charged, located in the nucleus. The number of protons (atomic number) defines the element.
- Neutrons — uncharged particles in the nucleus.
- Electrons — negatively charged, orbit the nucleus in an electron cloud; electrons have much smaller mass than protons and neutrons.
Atomic mass is approximately the sum of the masses of protons and neutrons.

Formed when two or more atoms are bonded together.
Atoms in a molecule are held together by chemical bonds, commonly covalent bonds (sharing electrons).
A molecule can consist of the same type of atom (elemental molecule) or different types of atoms:
- Example (same atoms): ozone (O3 — three oxygen atoms bonded).
- Example (different atoms): water (H2O — two hydrogen atoms bonded to one oxygen atom).
Note: a molecule is simply a bonded group of atoms; some molecules are also compounds (if they contain different elements).

A substance formed when two or more different elements are chemically combined.
For a substance to be a compound, the constituent elements must be different.
Compounds can be molecules (discrete bonded groups of atoms) or ionic lattices (extended ionic structures).
Example: sugar — a compound made of carbon, hydrogen, and oxygen.

Atom = single unit of an element.
Molecule = two or more atoms bonded together (same or different elements).
Compound = a molecule or substance made of two or more different elements chemically combined.