Question:
Consider the elements with atomic numbers \(Z=1\) to \(Z=20\). The number of elements with only one unpaired electron in their ground state is
Consider the elements with atomic numbers \(Z=1\) to \(Z=20\). The number of elements with only one unpaired electron in their ground state is
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Use electronic configuration and Hund's rule to count unpaired electrons in ground state.
Updated On: May 7, 2026
- \(10\)
- \(6\)
- \(8\)
- \(12\)
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The Correct Option is C
Solution and Explanation
We need to count elements from \(Z=1\) to \(Z=20\) having only one unpaired electron in the ground state.
Now check important electronic configurations.
Hydrogen:
\[
Z=1:\quad 1s^1
\]
It has one unpaired electron.
Lithium:
\[
Z=3:\quad 1s^2 2s^1
\]
It has one unpaired electron.
Boron:
\[
Z=5:\quad 1s^2 2s^2 2p^1
\]
It has one unpaired electron.
Fluorine:
\[
Z=9:\quad 1s^2 2s^2 2p^5
\]
It has one unpaired electron.
Sodium:
\[
Z=11:\quad [Ne]3s^1
\]
It has one unpaired electron.
Aluminium:
\[
Z=13:\quad [Ne]3s^2 3p^1
\]
It has one unpaired electron.
Chlorine:
\[
Z=17:\quad [Ne]3s^2 3p^5
\]
It has one unpaired electron.
Potassium:
\[
Z=19:\quad [Ar]4s^1
\]
It has one unpaired electron.
Thus, the elements are:
\[
H,\ Li,\ B,\ F,\ Na,\ Al,\ Cl,\ K.
\]
Total number is:
\[
8.
\]
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