What is an Isotope, Isotone and Isobar?

Most of us are only familiar with the term isotope and don’t know about isotones and isobars. Basically all these 3 terms are related in some sense and easy to remember, as single alphabet in these words indicates the difference. Before knowing about these terms, let’s remind that an atom is the basic unit of matter which is indivisible, but with the passage of time and research, the further division of atom into protons, neutrons and electrons was confirmed and even into quarks. So basically an atom consists of a nucleus, which consists of protons and neutrons, and out of this nucleus there are electrons which are in shells. Interior of a nucleus: neutron and proton is also termed as nucleons. Number of protons is called proton number or atomic number (Z) and the number of neutrons is called neutron number (N). The sum of neutron and protons is called mass number (A).

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In these 3 terms, “iso” means “same” and next to it, gives it a proper definition.

In isotope, the alphabet “P” indicates proton number, so in isotopes proton number is same.

In isotone, the alphabet “N” indicates number of neutrons, so isotones have same number of neutrons.

In isobar, the alphabet “A” indicates the mass number, so isobars have same mass number. Since mass number is indicated by “A”.

The key difference in these 3 terms is that isotopes are the “atoms of same chemical element”, which have same atomic number but different mass number i.e. neutron number is different. Whereas, isobars are the “atoms of different chemical elements”, which have same atomic masses. Lastly, isotones are those “atoms of different elements” which have same number of neutrons. Let’s now discuss all these terms, separately, in detail.


How Isotope, Isotone and Isobar Differ?

Isotope are those atoms of same chemical element, which have same number of protons, but different number of neutrons, thus mass number is also different. Since isotopes have same source element, so their chemical properties are also same but their physical properties alter from each other. There are almost 275 known isotopes of 81 stable elements. An element can have stable as well as radioactive isotopes (unstable). The examples of isotopes are given as follows:

Hydrogen: 1H1 (protium) , 1H2 (deuterium) , 1H3 (tritium)

Helium: 2He3, 2He4

Carbon: 6C12, 6C13, 6C14

Nitrogen: 7N14, 7N15

Sulfur: 16S32, 16S33 , 16S34 , 16S36 , these are stable isotopes of sulfur, unstable isotopes include S31, S35, S38, S44.

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Isobars are defined as the atoms of different chemical elements which have same mass number (A). Since atomic mass is sum of number of nucleons, so in isobars the number of nucleons is same. Since different chemical elements have different atomic numbers so, clearly, isobars have different atomic numbers.

How Isotope, Isotone and Isobar Differ?

According to Mattauch rule for isobar, if the 2 adjacent elements in periodic table have isotopes with same mass number i.e. they are isobars, then one of these elements will be radioactive and other will be stable. In case of 3 elements, 1st and last element will be stable and middle element will be radioactive.

Examples of isobars are given below:

  • Series 58: 26Fe58 , 27Ni58
  • Series 76: 32Ce76 , 34Se76
  • Series 40: 18Ar40 , 19K40 , 20Ca40
  • Series 24: 11Na24 , 12Mg24

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The atoms of different chemical elements which have same number of neutrons in nucleus. Atomic numbers of isotones are different, thus mass numbers are also different.

How Isotope, Isotone and Isobar Differ?

Examples of isotones are given below:

  • Series 20: 18Ar38 , 19K39 , 20Ca40
  • Series 50: 36Kr86 , 38Sr88 , 39Y89 , 40Zr90 , 42Mo92
  • Series 7: 5B12 , 6C13

There are no stable isotones for series 19, 21, 35, 39, 45, 61, 89, 115, 123, 127 and many more.

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In isobars and isotones, series number actually indicates the atomic number and neutron number, respectively, which are fixed in respective cases. For example in isotone series 50, each element will possess 50 number of neutrons.

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