Question

why do we need to take out the average atomic mass of isotopes of an element for computation ?

Answer 1

We saw in the unit atomic shorthand that each chemical element has its own unique atomic number .that means that all the atoms of any particular element must have the same number of protons .so any atom had two proton . It would be helium. But there is no reason why the number of neutrons cant vary .in fact , there are three types of hydrogen atom each containing a diff number of neutrons . We call these isotopes of hydrogen .so isotopes are atoms of the same element that have different masses.
I hope it help uh .....

Answer 2

Every element has an isotope.

During a reaction we do not know which element is to be used that is why we need to take out average atomic mass of element.