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

·        Atomic Number is the number of protons in the nucleus of an atom.

·        Mass Number is total number of protons plus neutrons in the nucleus of an atom.

·        Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.

·        Relative Atomic Mass (A_r) is the weighted average of the masses of all the stable isotopes of an element.  Relative Atomic masses are measured on a scale where the mass of 12C = 12 units.

Isotope		Atomic number	Mass number	Number of protons	Number of electrons	Number of neutrons
chlorine-35	35Cl	17	35	17	17	18
chlorine-37	37Cl	17	37	17	17	20

(note that the number of neutrons = mass number - atomic number)