In the neutrons, massive, neutrally charged, nuclear particles.
#Z,"the atomic number"# is the number of protons, massive, positively charged particles. For the neutral atom, the number of electrons, fundamental, negatively charged particles of negligible mass, is necessarily the same. (Why necessarily?) A given nucleus may contain varying numbers of neutrons, which gives rise to the existence of isotopes.
As an example, for hydrogen #Z=1#; and thus a hydrogen nucleus contains 1 proton; this is what defines the nucleus as hydrogen. A small percentage of hydrogen nuclei contain 1 proton AND 1 neutron, to give the deuterium isotope, which we would represent as #""^2H" (or as D)"#; a smaller percentage of hydrogen nuclei would contain 2 neutrons to give the tritium isotope #""^3H#.
All the isotopes have the same atomic number, and are thus the same element, but they have different masses. This can be very useful in labelling studies where the masses of each isotopomer are interrogated.