Isotopes:
It has been observed that many elements have nuclei which contain the same number of protons but different number of neutrons. Since the nuclei of the atoms have same number of protons, it means that they have same atomic number, Z, but different values of the mass number, A. Such atoms of the same element are called isotopes. Thus,
“Isotopes are atoms of the same element which have the same atomic number but different mass numbers”.
Since the atomic number of different isotopes of the same element is same, it means that they have same number of electrons and protons. The difference in their mass numbers is due to different number of neutrons present in their nuclei. e.g. Hydrogen has three isotopes. The hydrogen which is mostly abundant (99.985%) contains only 1 proton. It is also called protium. Rest of the hydrogen contains two isotopes, one containing 1 proton and 1 neutron called deuterium (2D1 or 2H1, 0.015%) and the other possessing 1 proton and 2 neutrons called tritium ( or 3T1). Tritium is found only in trace amounts on the earth. These have the same atomic number, 1, but different mass number 1, 2 and 3.
Similarly, chlorine has two isotopes having same atomic numb Z =17 whereas, their mass numbers are (17 proton, 18 neutrons) and 37 (17 protons and 20 neutrons. These isotopes do not have special names, but indicated by giving the value of A on the symbol for the element as Cl35 & Cl37.
Isobar:
Atoms of different elements having the same mass number but different atomic numbers are called isobars.
Since isobars have same mass number, therefore, the sum of protons and neutrons in the nucleus of each atom is the same. For example, 40Ar18 (18 protons, 22 neutrons), 40K19 (19 protons, 21 neutrons) and 40Ca20 (20 protons, 20 neutrons) are isobars because they have same mass number (40). .Isobars are atoms of different elements and hence they have different properties.
Isotones:
The isotones are atoms having same number of neutrons but different mass numbers are called isotones. For example 30Si14 (14 protons, 16 neutrons), 31P15 (15 protons, 16 neutrons) and 32S16 (16 protons, 16 neutrons) are isotones because all have 16 neutrons.
Mass Number = No. of protons + No. of neutrons
Mass number is generally represented by the letter A. Therefore, from the knowledge of atomic number and mass number of an element, the no. of electrons, protons and neutrons can be easily calculated.
Atomic Number (Z):
In 1913, Moseley determined the magnitude of the positive charge on the nucleus of an atom by studying the nature of X-rays emitted by different elements. The no. of positive charge carried by the nucleus of an atom is termed as the atomic number. Or, atomic number is the total number of either electrons or protons present in an atom. It is denoted by letter Z. Therefore,
Atomic no. (Z) = No. of protons = No. of electrons
We know,
Atomic mass (A) = Atomic no. (Z) + No. of neutrons (n)