The three isotopes of hydrogen

Hydrogen has three isotopes, and they are the clearest example there is — because hydrogen is small enough that you can see the whole difference at a glance.

They are called protium, deuterium and tritium.

Isotope	Protium	Deuterium	Tritium
Electrons	1	1	1
Protons	1	1	1
Neutrons	0	1	2
Atomic number	1	1	1
Mass number	1	2	3
Standard notation	¹₁H	²₁H	³₁H

Table 3.4 — the isotopes of hydrogen.

Figure to be added

Table 3.4's atomic models: three hydrogen atoms drawn side by side, each with one electron on a shell around the nucleus, and 0, 1 and 2 neutrons in the nucleus respectively.

What this shows

Read the rows in order and the point makes itself. The first four rows are identical across all three — one electron, one proton, atomic number 1. Only the neutrons move, and the mass number follows them: 1, 2, 3.