Hydrogen

Hydrogen has a mass number of one and an atomic number of one.

 Hydrogen atoms, therefore, contain only one proton and one electron.

No neutrons? No wonder they’re so light!

NOTE:

As shown above, hydrogen has no neutrons.

The number of protons defines an element, because it’s the only thing that remains constant; everything else can change.

Electrons can be transferred from one atom to another to make ions and the number of neutrons can also be altered to form an isotope (‘ice on top’).

It is the number of protons that defines the name of the atom.