# Structure of the Atom

#### Isobars

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:Isobars are the atoms of different elements having different atomic numbers but the same mass number (or same atomic mass).Isobars have different number of protons in their nuclei but the total number of nucleons (protons + neutrons) in them is the same. An example of isobars is argon,$=\begin{array}{c}\mathrm{40}\\ \mathrm{18}\end{array}$Ar and calcium $=\begin{array}{c}\mathrm{40}\\ \mathrm{20}\end{array}$Ca.This is because argon and calcium are atoms of different elements having different atomic numbers of 18 and 20 respectively but the same mass number of 40. The complete nuclear composition of the isobars $=\begin{array}{c}\mathrm{40}\\ \mathrm{18}\end{array}$Ar and $=\begin{array}{c}\mathrm{40}\\ \mathrm{20}\end{array}$Ca is given below :
IsobarProtonsNeutronsMass 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 answer the following questions :

## Notes

Structure of the Atom - Notes
1. Drawback of Rutherford’s Model of the Atom
Show Notes
2. Rutherford’s Experiment - Discovery of Nucleus
Show Notes
3. For the symbols H, D and T,tabulate three sub-atomic particles found in each of them.
Show Notes
4. 4.Isotopes of Neon.
Show Notes
5. Structure of The Atom - Study Points
Show Notes
6. 3.Isotopes of Oxygen.
Show Notes
Show Notes
8. Characteristics of a Neutron
Show Notes
9. Nucleus
Show Notes
10. Arrangement Of Electrons In The Atoms
Show Notes
11. Limitations of Rutherford's model of the atom
Show Notes
12. Characteristics of an Electron
Show Notes
13. (b) Valency of Magnesium
Show Notes
14. (d) Covalency of Nitrogen
Show Notes
15. All about Names of the Chemical Elements
Show Notes
16. Mass Number
Show Notes
17. How to learn naming Chemical Formulae?
Show Notes
18. 1. Isotopes of Hydrogen.
Show Notes
19. (d) Valency of Chlorine
Show Notes
20. Characteristics of a Proton
Show Notes
21. Comparison between Proton, Neutron and Electron
Show Notes
22. Electronic Configurations of First 20 Elements
Show Notes
23. Discovery of Neutron
Show Notes
24. Thomson's Model Of The Atom
Show Notes
25. (e) Valency of Oxygen
Show Notes
26. Rules for writing of distribution of electrons in various shells for the first 18 elements
Show Notes
27. Isotopes
Show Notes
28. 2.Covalency
Show Notes
29. Covalency of Oxygen
Show Notes
30. (a) Covalency of Hydrogen
Show Notes
31. Bohr's Model Of The Atom
Show Notes
32. Discovery Of Electron
Show Notes
33. Rutherford's Model Of The Atom
Show Notes
34. The Physical Properties of the Isotopes of an Element are Different
Show Notes
35. 2.Isotopes of Carbon.
Show Notes
36. Atomic Number
Show Notes
37. (a) Valency of Sodium
Show Notes
38. Charged Particles in Matter
Show Notes
39. Discovery of Proton
Show Notes
40. All the Isotopes of an Element Have Identical Chemical Properties
Show Notes
41. Valaency Of Elements
Show Notes
42. Relationship Between Mass Number and Atomic Number
Show Notes
43. (f) Valency of Nitrogen
Show Notes
44. Valaence Electrons (Or Valancy Electrons)
Show Notes
45. Electronic Configurations of Noble Gases (or Inert Gases)
Show Notes
46. Covalency of Chlorine
Show Notes
47. Cause of Chemical Combination
Show Notes
48. Reason for the Fractional Atomic Masses of Elements
Show Notes