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# Among $K{{O}_{2}}$, $AlO{{2}^{-}}$, $Ba{{O}_{2}}$ and $NO{{2}^{+}}$ unpaired electron is present in :A.$NO{{2}^{+}}$ and $Ba{{O}_{2}}$B.$K{{O}_{2}}$ and $AlO{{2}^{-}}$C.$K{{O}_{2}}$ onlyD.$Ba{{O}_{2}}$ only

Last updated date: 13th Jun 2024
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Hint:In order to solve this question we will first calculate the number of electrons each element has and then calculate the total number of electrons in the ions.
Then we will find out which ion has an unpaired number of electrons.

In the given question the ions $K{{O}_{2}}$, $AlO{{2}^{-}}$, $Ba{{O}_{2}}$ and $NO{{2}^{+}}$ are given and we are supposed to find out the one ion which has unpaired number of electrons present in it. So we will first individually calculate the number of electrons in each of these ions.
As we know that the number of electrons in potassium is $19$. And the oxygen has $8$ electrons in total. So the first molecule is $K{{O}_{2}}$ we will add all these electrons in order to get the total number of electrons in the molecule. We will multiply the number of electrons of oxygen by two because there are two atoms of oxygen involved.
So, $19+8\left( 2 \right)=35$
So the total number of electrons in $K{{O}_{2}}$ is $35$
Now we will calculate the number of1 electrons in $AlO{{2}^{-}}$now this is an ion, so while calculating the number of electrons we will add one extra electron because of the negative charge.
We know that aluminium has $13$ electrons, and oxygen has 8. So the total number of electrons is,
$13+8\left( 2 \right)+1=30$
Now in case of $Ba{{O}_{2}}$ the number of electrons would be, $56$ for barium and oxygen will still remain the same. Now this is not an ion, so we will not add any extra electrons while calculating the total number of electrons.
$56+8\left( 2 \right)=72$
Now in the case of $NO{{2}^{+}}$ the number of electrons of nitrogen is $7$ and that of oxygen is$8$. Now notice it is a cation and not anion, so we will not add an electron we will subtract it from the total number of electron
$7+8\left( 2 \right)-1=22$
Now we know that generally, when the total number of electrons is odd, that is because they have an unpaired number of electrons.
Since only $K{{O}_{2}}$ has an odd number of electrons, so it has an unpaired number of electrons.
So the correct option is C.

Note:
While filling up electrons in an orbital, first they singly occupy all the orbitals then the electron starts pairing. This rule is called Hund’s rule.
We can tell about the magnetic properties of an element or a compound just by knowing if they have paired or unpaired electrons. If they have any unpaired electrons they’ll be paramagnetic in nature.