As you might have read in your earlier classes that an atom consists of several fundamental particles: electrons, protons and neutrons. An electron is negatively charged and a proton is positively charged particle. Number of electrons and protons in an atom is equal. Since charge on an electron is equal and opposite to charge of a proton, therefore, an atom is electrically neutral. Protons remain in the nucleus in the centre of the atom and nucleus is surrounded by negatively charged electrons.

The number of protons in the nucleus is called atomic number, and is denoted by Z. There are also neutral particles in the nucleus and they are called neutrons. Mass of a proton is nearly equal to the mass of neutron. Total mass of nucleus is equal to the sum of masses of protons and neutrons. The total number of protons and neutrons is called mass number or the nucleons number denoted by A. By convention, atomic number is written at the bottom left corner of the symbol of the atom and mass number is written at the top left corner. For example, we write, ⁴₂He, ⁷₃Li and ¹²₆Cfor helium, lithium and carbon respectively. The symbol ¹²₆Cindicates that there is a total of 12 particles (nucleons) in the nucleus of carbon atom, 6 of which are protons. Thus, there must be 12 – 6 = 6 neutrons. Similarly, ¹⁶₈O has 8 protons and 8 electrons and there are 8 neutrons. Also atomic number, Z differentiates the atom of one element from the atoms of another. Also an element may be defined as a substance whose atoms have the same atomic number. Thus, all atoms of an element have nuclei containing the same number of protons and having the same charge. But the nuclei of all the atoms of a given element do not necessarily contain the same number of neutrons. For example, atoms of oxygen, found in nature have the same number of protons which makes it different from other elements, but their neutrons are different. This is the reason that the masses of the atoms of the same elements are different. For example, one type of oxygen atom contains 8 protons and 8 neutrons in the nucleus, second type 8 protons and 9 neutrons and third type contains 8 proton and 10 neutrons. We represent them as ¹⁶₈O, ¹⁷₈O and ¹⁸₈O. Atoms of an element that have same atomic number(Z) but different mass number(A) are called isotopes.