We have just studied isotopes which are atoms of the same element having the same atomic number but different mass numbers.It is, however, also possible that in some cases, the atoms of different elements having different atomic numbers may have the same mass number. Such atoms are called isobars. We can now say that:

Isobar | Protons | Neutrons | Mass number |
---|---|---|---|

$=\begin{array}{c}\mathrm{40}\\ \mathrm{18}\end{array}$Ar $=\begin{array}{c}\mathrm{40}\\ \mathrm{20}\end{array}$Ca | 18 20 | 22 20 | 18+22=40 20+20=40 |

Here is another example of isobars.The radioactive sodium,$=\begin{array}{c}\mathrm{24}\\ \mathrm{11}\end{array}$Na,and magnesium $=\begin{array}{c}\mathrm{24}\\ \mathrm{12}\end{array}$Mg,are isobars.This is because radioactive sodium and magnesium are different elements having different atomic numbers of 11 and 12 respectively but the same mass number of 24.

In order to find out isobars from among a number of given species, we should look at their atomic numbers and mass numbers. The atomic species having different atomic numbers but same mass number will be isobars. We will now solve some problems based on isobars.

Sample Problem 1. Which two of the following atomic species are isotopes of each other and which two are isobars ?

$=\begin{array}{c}\mathrm{231}\\ \mathrm{90}\end{array}$Z,$=\begin{array}{c}\mathrm{230}\\ \mathrm{91}\end{array}$Z,$=\begin{array}{c}\mathrm{230}\\ \mathrm{88}\end{array}$Z,$=\begin{array}{c}\mathrm{233}\\ \mathrm{90}\end{array}$Z

Solution (a).The isotopes of an element have the same atomic number but different mass numbers.The lower figures in the above given symbols indicate the atomic numbers.Now,in this case there are two atoms having the same atomic number of 90. So, the two isotopes will be :$=\begin{array}{c}\mathrm{231}\\ \mathrm{90}\end{array}$Z and $=\begin{array}{c}\mathrm{233}\\ \mathrm{90}\end{array}$Z

(b) The isobars have different atomic numbers but same mass numbers. The upper figures in the given symbols indicate the mass numbers. In this case there are two atoms having the same mass number of 230.So, the two isobars will be :

$=\begin{array}{c}\mathrm{230}\\ \mathrm{91}\end{array}$Z and $=\begin{array}{c}\mathrm{230}\\ \mathrm{88}\end{array}$Z

Sample Problem 2.Write the electronic configurations of any one pair of (a) isotopes,and (b) isobars.

Solution, (a) A pair of isotopes of chlorine is $=\begin{array}{c}\mathrm{35}\\ \mathrm{17}\end{array}$Cl and $=\begin{array}{c}\mathrm{37}\\ \mathrm{17}\end{array}$Cl.The atomic number of both the isotopes is the same, 17. So, the electronic configuration of both these isotopes will be 2,8,7.

(b) A pair of isobars is $=\begin{array}{c}\mathrm{40}\\ \mathrm{18}\end{array}$Ar and $=\begin{array}{c}\mathrm{40}\\ \mathrm{20}\end{array}$Ca.The atomic number of argon (Ar) is 18, so its electronic configuration will be 2,8,8. The atomic number of calcium (Ca) is 20, so its electronic configuration will be 2, 8, 8, 2.

We are now in a position to

1. Drawback of Rutherford’s Model of the Atom

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2. Rutherford’s Experiment - Discovery of Nucleus

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3. For the symbols H, D and T,tabulate three sub-atomic particles found in each of them.

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4. 4.Isotopes of Neon.

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5. Structure of The Atom - Study Points

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6. 3.Isotopes of Oxygen.

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7. Radioactive Isotopes

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8. Characteristics of a Neutron

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9. Nucleus

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10. Arrangement Of Electrons In The Atoms

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11. Limitations of Rutherford's model of the atom

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12. Characteristics of an Electron

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13. (b) Valency of Magnesium

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14. (d) Covalency of Nitrogen

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15. All about Names of the Chemical Elements

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16. Mass Number

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17. How to learn naming Chemical Formulae?

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18. 1. Isotopes of Hydrogen.

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19. (d) Valency of Chlorine

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20. Characteristics of a Proton

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21. Comparison between Proton, Neutron and Electron

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22. Electronic Configurations of First 20 Elements

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23. Discovery of Neutron

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24. Thomson's Model Of The Atom

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25. (e) Valency of Oxygen

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26. Rules for writing of distribution of electrons in various shells for the first 18 elements

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27. Isotopes

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28. 2.Covalency

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29. Covalency of Oxygen

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30. (a) Covalency of Hydrogen

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31. Bohr's Model Of The Atom

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32. Discovery Of Electron

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33. Rutherford's Model Of The Atom

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34. The Physical Properties of the Isotopes of an Element are Different

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35. 2.Isotopes of Carbon.

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36. Atomic Number

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37. (a) Valency of Sodium

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38. Charged Particles in Matter

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39. Discovery of Proton

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40. All the Isotopes of an Element Have Identical Chemical Properties

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41. Valaency Of Elements

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42. Relationship Between Mass Number and Atomic Number

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43. (f) Valency of Nitrogen

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44. Valaence Electrons (Or Valancy Electrons)

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45. Electronic Configurations of Noble Gases (or Inert Gases)

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46. Covalency of Chlorine

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47. Cause of Chemical Combination

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48. Reason for the Fractional Atomic Masses of Elements

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49. Applications of Radioactive Isotopes

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50. Isobars

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