1) The electron of an atom controls the overall charge  and the chemical properties of the atom or a compounds. WHY?
     Only the electron can be removed (to give cations) or added (to give anions) to an atom, which completely change the reactivity of the atom.  For example, K atom is extremely active (see Fig. 2.4 on P. 46); however, K ion (i.e. K⁺) is very inert and is one of the most essential elements in the biological systems.

2) Atomic number is not defined as the number of electrons because electrons of an atom can be easily removed or added which would result in different atomic numbers for the atom and its cations or anions!

3) actual mass of an atom
= mass of proton (1.67 x 10^–24 g) x No. proton
+ mass of electrons (9.11 x 10^–28 g) x No. electrons
+ mass of neutron (~mass of proton) x No. neutrons

This mass is rarely used because it is too small to be conveniently used!

4) Simply change the numbers in the question for oxygen to variables, you get the answer as (aX/100 + bY/100 + cZ/100)