Inside an atom.

Step 1 of 5

Three particles

All matter, from a glass of water to a lump of iron, is made of atoms, and the atoms themselves are made of only three kinds of particle. At the centre of the atom is the nucleus, holding protons, which are positively charged, and neutrons, which have no charge. Around the nucleus move the electrons, which are negatively charged.

Particle	Where	Charge	Relative mass
Proton	in the nucleus	$+1$⁠	$1$⁠
Neutron	in the nucleus	$0$⁠	$1$⁠
Electron	around the nucleus	$-1$⁠	$\tfrac{1}{1836}$⁠

The Scene shows a carbon atom: six protons (orange) and six neutrons (pale) packed in the nucleus, and six electrons on rings around it. Change Protons and see that a whole atom has one electron for every proton: the charges cancel, and the atom is neutral.

Step 2 of 5

The atomic number

What makes an atom oxygen rather than carbon? Only the number of protons in its nucleus. That number is the atomic number, written $Z$⁠. Every oxygen atom in the world has eight protons, and every carbon atom has six. An element is a substance whose atoms all have the same atomic number, and the periodic table puts the elements in order of $Z$⁠: hydrogen is $1$⁠, helium $2$⁠, and so on.

The Scene now shows oxygen, $Z = 8$⁠. Try adding one proton: the atom is no longer oxygen.

Step 3 of 5

The mass number

Protons and neutrons have almost the same mass, and electrons are almost 2000 times lighter. So an atom’s mass is set by the number of particles in its nucleus. That number is the mass number, $A$⁠:

$A = Z + N$⁠

where $N$⁠ is the number of neutrons. So to find how many neutrons an atom has, subtract the atomic number from the mass number: $N = A - Z$⁠.

The Scene now shows aluminium, with the protons locked. This atom’s mass number is $27$⁠. Work out how many neutrons its nucleus holds, and count them in the Scene if you like.

Step 4 of 5

Writing an atom

Chemists write an atom as the element’s symbol with the mass number at the top left and the atomic number at the bottom left. The aluminium from the last step is $^{27}_{13}\mathrm{Al}$⁠, and sodium is $^{23}_{11}\mathrm{Na}$⁠. All three numbers can be read from the symbol: $Z$⁠ protons, $A - Z$⁠ neutrons, and in a neutral atom $Z$⁠ electrons.

The Scene now shows potassium, $^{39}_{19}\mathrm{K}$⁠.

Step 5 of 5

Finding an atom by its mass

The Scene draws each element’s most common atom. Carbon’s has six neutrons, so its mass number is $12$⁠; oxygen’s has eight, and its mass number is $16$⁠. The heavier the atom, the more neutrons it usually has beyond its protons. Use Protons to find an atom by its mass number.