Write a symbolic notation of an isotope of an element having 8 protons 8 electrons and 10 neutrons

                       ¹⁸₈O

Explanation:

                     The symbolic notation of an isotope of an element is generally written as;

                                                  ᵃₙX

Where;

                 a  =  Mass Number

                 n  =  Atomic Number

                 X  =  Element Symbol;

Mass number is the sum of neutrons and protons present in the nucleus. It is given as,

Mass Number  =  No. of Protons  +  No. of Neutrons

As Given

                 Mass  Number  =  8 + 10

                 Mass  Number  =  a  =  18

Also,

Atomic Number is defined as the number of protons contained in the nucleus of an atom. Hence, in this case;

                 Number of Protons = Atomic Number = n = 8

Also, the element with atomic number 8 in periodic table is Oxygen. Hence;

                 X  =  O

Therefore, The symbolic notation for given element is ¹⁸₈O.

the formula mass of a molecule or a compound is the sum of the atomic weights of the atoms in its empirical formula

(that's what i know about formula mass)