Explain why every whole number is also a rational number but not every rational number is a whole number

The definition of rational means it can be put into fraction form. Every whole number can be put into fraction form. However, not every fraction is a whole number (A number with no decimal)

A rational number can be written as a/b where a and b are  integers  and a/b is at its lowest form.

So  a whole number 5 is ratiional  - it can be written as 5/1   but  fractions are also rational,   for example 2/3 and 3/4.