An atom has 8 protons, 8 neutrons, and 8 electrons. another isotope of the same element might have

Isotopes are atoms of the same element that have the same number of protons and electrons but different number of neutrons.
Masses of atoms are the sum of protons and neutrons. Therefore when the number of neutrons change but protons are the same, then the masses of the atoms change.
isotope 1 has 8 protons , 8 electrons and 8 neutrons.
isotope 2 has 8 protons, 8 electrons and 10 neutrons

mass of isotope 1 is 16
and mass of isotope 2 is 18

D. most models represent a real world object in a simpler way