Chemistry: The Central Science (13th Edition)

The element with the electron configuration is chromium ($Cr$). There are 6 unpaired electrons.
*Strategy: 1) Find out the number of electrons it has by summing all the electrons in different subshells. 2) The number of electrons will be equal to the atomic number of the element. 3) Look up the periodic table to find out which element it is. 1) $[Ar]4s^13d^5$ - An $Ar$ atom has 18 electrons. - Subshell $4s$: 1 electron - Subshell $3d$: 5 electrons In total, the element has 24 electrons. 2) Since the number of electrons is equal to the atomic number of the element, the atomic number of the element is 24. 3) The element that has the atomic number 24 is chromium ($Cr$). 4) All the inner-shell electrons (those in the $Ar$) are paired. We know that subshell s has 1 orbital. Here only 1 electron occupies the only orbital in subshell $4s$, so it is unpaired. D-subshell has 5 orbitals, each can carry 2 electrons in maximum. Here there are 5 electrons that need to occupy. According to Hund's rule, these 5 electrons would each occupy a different orbital, so there would be minimum electron-electron repulsions. In other words, there are 5 one-electron orbital in subshell $3d$. That means there are 5 more unpaired electrons. Overall, there are 6 unpaired electrons.