Question

Which first-row transition metal has the most unpaired electrons?

Answer 1

Chromium. Is it also the most paramagnetic? Why or why not?

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The only candidates would be in the center of the transition metals, i.e. the ones with close to five `3d` electrons (`"Cr", "Mn", "Fe"`).

Iron has the electron configuration:

`[Ar] overbrace(3d^color(red)(4))^"unpaired" overbrace(3d^2)^" already paired" 4s^2`

`underbrace(ul(uarr darr)" "ul(uarr color(white)(darr))" "ul(uarr color(white)(darr))" "ul(uarr color(white)(darr))" "ul(uarr color(white)(darr)))`
`" "" "" "" "" "" "" "3d`

`ul(uarr darr)`
`4s`

Manganese has the electron configuration:

`[Ar] overbrace(3d^color(red)(5))^"unpaired" 4s^2`

`underbrace(ul(uarr color(white)(darr))" "ul(uarr color(white)(darr))" "ul(uarr color(white)(darr))" "ul(uarr color(white)(darr))" "ul(uarr color(white)(darr)))`
`" "" "" "" "" "" "" "3d`

`ul(uarr darr)`
`4s`

But chromium has...

`[Ar] overbrace(3d^color(red)(5) 4s^color(red)(1))^"unpaired"`

`underbrace(ul(uarr color(white)(darr))" "ul(uarr color(white)(darr))" "ul(uarr color(white)(darr))" "ul(uarr color(white)(darr))" "ul(uarr color(white)(darr)))`
`" "" "" "" "" "" "" "3d`

`ul(uarr color(white)(darr))`
`4s`

`6 > 5 > 4`, so...