Unpaired electrons are those electrons in an atom which occur alone in the orbitals.
Let me explain this, atoms have orbits which are called KLMN and then these orbits have subshells which are called 1s, 2s, 2p etc. Further these subshells have orbitals each of which can contain either 1 or 2 electrons. So if an orbital have one electron instead of 2, that single electron will be counted as unpaired electron.
|2 in 2s|
3 in 2p
|2 in 3s|
3 in 3p
5 in 3d
|2 in 4s|
3 in 4p
5 in 4d
7 in 4f
Best way to determine how many unpaired electrons are there in an atom is to just write it’s electron configuration and then draw its orbital diagram.
From orbital diagram we can easily calculate how many unpaired electrons are there.
So let’s first write electron configuration of Iron and then draw it’s orbital diagram.
Writing electron configuration of Iron
Iron atom in total have 26 electrons, to write down the electron configuration of iron, the first two electrons would go into the 1s orbital. Because the 1s orbital can only accommodate two electrons, next two electrons are placed in the 2s orbital, next 6 electrons are placed in 2p orbital. Out of 16 remaining electrons, 2 would go into 3s orbital, 6 electrons are placed in 3p orbitals, two are placed in 4s orbital and remaining 6 electrons are placed in 3d orbital. Putting all this together, electron configuration of Iron is 1s2 2s2 2p6 3s2 3p6 3d6 4s2. Which can also be written as [Ar] 3d6 4s2 where [Ar] represents electron configuration of noble gas Argon.
If your interested in knowing what’s full process of figuring out electron configuration of Iron, you can check this article – Writing Electron Configuration of Iron.
Drawing Orbital Diagram of Iron
So electron configuration of iron is 1s2 2s2 2p6 3s2 3p6 3d6 4s2 let’s draw orbital diagram now. But before doing this we need to keep in mind that as per Aufbau’s Principle increasing order of energy of different subshells is 1s < 2s < 2p < 3s < 3p.
Therefore in orbital diagram 1s should be written at bottom showing minimum energy while 3p should be written at top showing maximum energy.
Moreover for filling up these orbitals we need to follow a rule called Hund’s Rule of Multiplicity which state that pairing of electrons in orbitals start once each orbital in that subshell have one electron.
From above orbital diagram its clear that out of total 26 electrons in Iron atom, 22 electrons are paired in different orbitals and there are just 4 unpaired electrons. All these four unpaired electrons are located in 4 different orbitals of 3d subshell.
|Subshell||Number of Orbitals||Number of electrons||Paired or Unpaired|
|2p||3||6||Each of three orbitals|
have paired electrons
|3p||3||6||Each of three orbitals|
have paired electrons
|3d||5||6||4 Unpaired electrons in |
four different orbitals
2 electrons are paired in one orbital
Based upon electron configuration 1s2 2s2 2p6 3s2 3p6 3d6 4s2 and orbital diagram of iron, number of unpaired electrons in iron is 4 and these electrons are located in four different orbitals of 3d subshell.