An element is represented by ${}_Z{X}^A$, where Z is atomic number which is also equal to number of proton, A is mass number and X is symbol of the element.
Mass number (A) = Number of protons (Z) + Number of neutrons.
Isotopes: Atoms having same number of proton
Isobar: Elements having same mass number
Isotones: Elements having same number of neutron (A-Z)

The sum of the number of electrons, protons and neutrons in the three isotopes ${(}^{16}O{,}^{17}O{,}^{18}O)$ of oxygen is: