Difference between Isotopes, Isobars and Isotones

In this lesson , the Difference between Isotopes, Isobar and Isotones will be discussed with some Common Examples for all types.

What are Isotopes?

Isotopes may be defined as:

(1) The atoms of an element which have the same number of protons and different number of neutrons are called Isotopes.

(2) The atoms of an element which have the same atomic number but different atomic masses or mass numbers.

** Since isotopes of an element have the same atomic number, each of these contains equal number of protons.

** They have different atomic masses which is accounted for by the different number of neutrons present in the nucleus.

** Thus the isotopes of an element are characterised by different number of neutrons in the nucleus.

Examples of Isotopes

** Isotopes of Hydrogen: There are three isotopes of hydrogen : protium ¹H₁, deuterium ²H₁ or D , and tritium ³H₁ or T .

** Isotopes of Neon: Neon has been found to consist of three isotopes : ²⁰Ne₁₀, ²¹Ne₁₀, ²²Ne₁₀ 

** Isotopes of Oxygen: Oxygen has three isotopes : ¹⁶O₈ , ¹⁷O₈ , ¹⁸O₈ 

** Isotopes of Chlorine: Chlorine is a mixture of two isotopes : ³⁵Cl_17 ,³⁵Cl₁₇

** Isotopes of Uranium: There are three isotopes of uranium : ²³⁴U₉₂ , ²³⁵U_{92 ,} ²³⁸U_{92 ,} 

** Isotopes of Carbon: There are three isotopes of uranium : ¹²C₆ ,¹³C_{6 ,} ¹⁴C₆

What are Isobars?

** Isobars are The atoms which have the same mass number but different atomic numbers are called isobars.

** The word isobar meaning ‘equally heavy’ is taken from the Greek isos = equal, and barys = heavy.

Examples of isobars

** For example, ⁴⁰ Ar₁₈ , ⁴⁰K₁₉ , ⁴⁰Ca₂₀ are isobaric atoms. Similarly, ²³⁵U₉₂, ²³⁵Np₉₃, ²³⁵Pu₉₄ are isobars.

** Since isobars have the same mass number, the number of protons plus neutrons in the nucleus in each of these is equal.

** The number of protons being given by atomic number (Z), the number of neutrons is, therefore, (A – Z) where A is the mass number.

** The number of extranuclear electrons is equal to (Z).

** Thus the atomic structure of the isobars ⁴⁰Ar₁₈ , ⁴⁰K₁₉ and ⁴⁰Ca₂₀ is shown below:

What are Isotones?

** Isotones are : Atoms which have different atomic number and different atomic masses but the same number of neutrons .

** Isotones are different elements having entirely different atomic structure. They have different physical and chemical properties.

Examples of isotones:

(1) ¹⁴C₆ , ¹⁵N₇ , ¹⁶O₈ are isotones since each contains eight neutrons

(2) ³⁰Si₁₄ , ³¹P₁₅ , ³²S₁₆ are isotones. Each contains sixteen neutrons.

(3) Some other examples of isotones are :

Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolour edition.

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