Isotopes, isobars, and isotones
In nature, there are atoms of a particular element whose nuclei have same number of protons but different number of neutrons. These kinds of atoms are called isotopes. In other words, isotopes are atoms of the same element having same atomic number Z, but different mass number A. For example, hydrogen has three isotopes and they are represented as 11H (hydrogen), 21H (deuterium),and 31H (tritium). Note that all the three nuclei have one proton and, hydrogen has no neutron, deuterium has 1 neutron and tritium has 2 neutrons.
The number of isotopes for the particular element and their relative abundances (percentage) vary with each element. For example, carbon has four main isotopes: 116C , 126C , 136C and 146C . But in nature, the percentage of 126C is approximately 98.9%, that of 136C is 1.1% and that of 146C is 0.0001%. The other carbon isotope 116C , do not occur naturally and it can be produced only in nuclear reactions in the laboratory or by cosmic rays.
The chemical properties of any atom are determined only by electrons, the isotopes of any element have same electronic structure and same chemical properties. So the isotopes of the same element are placed in the same location in the periodic table.
Isobars are the atoms of different elements having the same mass number A, but different atomic number Z. In other words, isobars are the atoms of different chemical element which has same number of nucleon. For example 4016S 4017Cl , 4018Ar , 4019K and 4020Ca are isobars having same mass number 40 and different atomic number. Unlike isotopes, isobars are chemically different elements. They have different physical and chemical properties.
Isotones are the atoms of different elements having same number of neutrons. 125B and 136C are examples of isotones which 7 neutrons.