#thornleigh

In a Atom we find we three sub-atomic particles, The Proton, Neutron and Electron. We need to know two things per sub-atomic particle, its charge and its weight.

Sub-Atomic Particle ------ Relative Mass  ------ Relative Charge

Proton                              1                               +1

Electron                            0                               -1

Neutron                            1                               0 

Atomic Number and Mass Number

The Mass number is the total number of Protons and Neutrons In one atom of an element

The Atomic number is the number of protons in a atom (Atoms are uncharged so this number is also the number of electrons)

Isotopes

Isotopes are atoms of the same element with the name number of protons but different number of neutrons. For example 35Cl and 37Cl are isotopes because they are both chlorine but 37Cl has two extra neutrons.

Because of these differing mass numbers, isotopes will have different physical properties(boiling point, melting point) but identical chemical properties

The percentage of each isotope that naturally occurs on the earth is referred to as its relative isotopic abundance 

Relative Atomic Mass, Ar

We use 12C as a standard to measure the relative atomic mass of all other elements. Its relative mass is given as 12 exactly, all other atoms are compared to this one

Relative Atomic Mass (Ar) can be defined as 

Ar=Average mass of an atom*12/Mass of one atom of Carbon 12