Chemistry

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{matrix} 40 \\ 18 \end{matrix}

Ar and calcium

= \begin{matrix} 40 \\ 20 \end{matrix}

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{matrix} 40 \\ 18 \end{matrix}

Ar and

= \begin{matrix} 40 \\ 20 \end{matrix}

Ca is given below :

Isobar	Protons	Neutrons	Mass number
$= \begin{matrix} 40 \\ 18 \end{matrix}$ Ar $= \begin{matrix} 40 \\ 20 \end{matrix}$ Ca	18 20	22 20	18+22=40 20+20=40

Here is another example of isobars.The radioactive sodium,

= \begin{matrix} 24 \\ 11 \end{matrix}

Na,and magnesium

= \begin{matrix} 24 \\ 12 \end{matrix}

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{matrix} 231 \\ 90 \end{matrix}

= \begin{matrix} 230 \\ 91 \end{matrix}

= \begin{matrix} 230 \\ 88 \end{matrix}

= \begin{matrix} 233 \\ 90 \end{matrix}

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{matrix} 231 \\ 90 \end{matrix}

Z and

= \begin{matrix} 233 \\ 90 \end{matrix}

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{matrix} 230 \\ 91 \end{matrix}

Z and

= \begin{matrix} 230 \\ 88 \end{matrix}

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{matrix} 35 \\ 17 \end{matrix}

Cl and

= \begin{matrix} 37 \\ 17 \end{matrix}

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{matrix} 40 \\ 18 \end{matrix}

Ar and

= \begin{matrix} 40 \\ 20 \end{matrix}

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