Particles of Matter
Protons and neutrons are the smallest particles of matter. Protons and neutrons have the same mass.
Hydrogen is element 1. There are two stable isotopes of hydrogen, hydrogen-1 and hydrogen-2. There is also one unstable isotope of hydrogen, hydrogen-3.
A hydrogen-1 atom consists of a single proton.
A hydrogen-2 atom is made up of a proton and a neutron. A hydrogen-2 atom is known as a deuton.
A hydrogen-3 atom is made up of one proton and two neutrons. A hydrogen-3 atom is known as a triton.
Helium is element 2. There are two stable isotopes of helium, helium-3 and helium-4. A helium-3 atom is made up of a proton and a deuton. A helium-4 atom is made up of two deutons.
Lithium is element 3. There are two stable isotopes of lithium, lithium-6 and lithium-7. A lithium-6 atom is made up of three deutons. A lithium-7 atom is made up of two deutons and one triton.
Atoms are made up of deutons and tritons (except for hydrogen-1 atoms and helium-3 atoms, which contain single protons). The total number of deutons and tritons in an atom of an element is equal to the atomic number of the element.