Atom vs Molecule: What Is the Difference?
An atom is the smallest piece of an element. A molecule is two or more atoms joined by chemical bonds. One oxygen atom on its own is an atom. Two oxygen atoms bonded together make an oxygen molecule, O2, the oxygen you breathe.
Atom vs molecule at a glance
| Atom | Molecule | |
|---|---|---|
| What it is | The smallest piece of an element | Two or more atoms joined by chemical bonds |
| Made of | Protons, neutrons and electrons | Atoms |
| Example | One oxygen atom, O | An oxygen molecule, O2, or water, H2O |
| Size | About 0.1 to 0.5 nm across | From about 0.2 nm to many nanometres |
| Kinds | 118 known elements | Millions of different molecules |
| Can be split by chemistry | No | Yes, into its atoms |
Inside an atom
Every atom has a tiny nucleus of protons and neutrons, with electrons moving around it. The number of protons decides which element it is: 1 for hydrogen, 6 for carbon, 8 for oxygen. Almost all of an atom is empty space. If the nucleus were the size of a pea, the atom would be the size of a football stadium.
From atoms to molecules
Atoms bond by sharing or moving electrons. Two oxygen atoms share two pairs of electrons to make O2. Add two hydrogen atoms to one oxygen and you get water, H2O. A molecule made of different elements is also a compound. Some substances, such as table salt, are made of ions in a lattice rather than separate molecules.
Quick facts
- Noble gases such as helium and neon exist as single atoms, not molecules.
- A single drop of water holds around 1.7 sextillion molecules, 17 followed by 20 zeros.
- Ozone, O3, is a molecule of three oxygen atoms.
- The word atom comes from the Greek for uncuttable.
- DNA molecules can contain billions of atoms.
See them in 3D
Atom vs molecule questions
What is the difference between an atom and a molecule?
An atom is the smallest piece of an element. A molecule is two or more atoms chemically bonded together.
Is oxygen an atom or a molecule?
Both words are used. A single O is an oxygen atom. The oxygen gas we breathe is made of O2 molecules, two oxygen atoms bonded together.
Is water an atom or a molecule?
A molecule, made of two hydrogen atoms and one oxygen atom.
Are all molecules compounds?
No. A molecule of one element, such as O2, is not a compound. A molecule with two or more different elements, such as H2O, is.
What is smaller than an atom?
Protons, neutrons and electrons, and protons and neutrons are made of even smaller quarks.
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