Isotopes, Isobars and Isotones in Chemistry

The elements having same atomic number but different mass number. They have the same atomic number because the number of protons inside their nuclei remains the same. The difference in their mass number is due to the difference in their number of neutrons.

Since they are neutral isotopes are elements having same number of electrons, which make them to possess identical chemical properties. Let us see some examples ₁H¹, ₁H², ₁H³ are all isotopes of hydrogen. They all have their atomic number to be unity but the number of neutrons are 0, 1, 2 and z respectively. ₁₇Cl³⁷, ₁₇Cl³⁵ are isotopes of chlorine. They have 17 protons in the nucleus but have number of neutrons equal to 20 and 18 respectively. Practically every element consists of a mixture of several isotopes. The relative abundance of different isotopes differs from element to element. For example chlorine is composed of two isotopes of masses 34.98U and 36.98U, which are nearly integral multiples of the mass of hydrogen atom. Their relative abundances are 75.4 and 24.6 percent respectively. Mass of natural chlorine atom can be found as

= 35.47.

The isotope can occur either naturally or can be produced artificially in the laboratory.

Isobars

Isotopes are chemically same and physically different. But the converse is true in isobars. That is isobars are elements, which are chemically different but physically same. So, isobars are atoms of different elements having the same atomic mass but different atomic number. Since their number of electrons is different, their chemical properties are different. The light nuclei have unstable isobars. Heavy nuclei have stable isobars and these occur in pairs. Suppose the number of protons of one isobar matches with that of another they are called as mirror-nuclides of each other.

Examples of isobars are

Since isobars are different elements they appear in different places in the periodic table.

Isotones

Isotones are elements having the same number of neutrons. Examples of isotones are Chlorine - 37 and Potassium - 39. Both have 20 neutrons in their nuclei.