1.11 understand the terms atomic number, mass number, isotopes and reptile atomic mass (Ar)

Atomic number - The number of protons (also the number of electrons)

Mass number - number of protons + neutrons (the number of stuff in the nucleus)

Isotopes - different atomic forms of the same element, which have the same number of protons but a different number of neutrons

Relative atomic mass (Ar) - the mass of one atom of the element (/compound)