Every atom is made of three particles. These are proton, neutron and electron. The number of protons in the atom is equal to the number of electrons as these are electrically charged particles and carry an equal but opposite charge; and atoms are electrically neutral in the natural state. The element of an atom is decided by the number of protons (which is the same as the number of electrons) in the atom. The number of protons in the atom of each element is unique, i.e. the atoms of different elements can only have a common number of neutrons.
Compared to the mass of the neutron and the proton which is approximately the same the mass of the electron is negligible. Atoms of the same element can differ in their mass which is the case if the number of neutrons in the atoms is not the same. These atoms with different mass are called isotopes.