1.12 calculate the relative atomic mass of an element from the relative abundances of its isotopes

There is variation is the relative atomic mass (mass of one atom) due to isotopes - different isotopes will weigh more due to more or less neutrons

It is calculated using this equation...

(% of isotope x its mass) + (% of isotope x its mass) / 100

this looks very confusing, but heres how to apply it to an element...

Example (chlorine)

75% of all chlorine has a mass of 35 and the rest (25%) has a mass of 37

now substitute it into the equation above...

(75 x 35) + (25 x 37) / 100 = 35.5

NOTE: do not worry about learning all the isotopes for every element, in the exam, should they pose a question like this, they will give you the % and masses.