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

The atomic number is the number of protons the element has. The mass number is the total number of protons and neutrons in the nucleus. Isotopes are different forms of the same element that have a different number of neutrons (an element ALWAYS has the same number of protons.) The relative atomic mass is the ratio of the average mass of an atom of a specific element to a twelfth of the mass of a carbon-12 atom.