A) Define: i) Atomic number ii) Mass number b) Calculate the relative atomic mass of element X given its isotopes and their abundances. c) Define isotopes.

Isotope X1: Mass number = 35, Abundance = 75%
Isotope X2: Mass number = 37, Abundance = 25%

Relative atomic mass $= \sum (\text{isotope mass} \times \text{fractional abundance})$

The relative atomic mass of element X is $\boxed{\text{35.5}}$.

6. c) Define the term isotopes.
Isotopes are atoms of the same element that have the same number of protons (atomic number) but different numbers of neutrons, resulting in different mass numbers.