## Chemistry: The Central Science (13th Edition)

The condensed electron configuration of $Br$ is $$Br: [Ar]4s^23d^{10}4p^5$$ There is 1 unpaired electron.
1) The nearest noble gas of lower atomic number of $Br$ is $Ar$. 2) Looking the periodic table, the atomic number of $Br$ is 35, which means it has 35 electrons. The last rows ends with the atomic number 18, so $Br$ has 18 inner-shell electrons. That leaves it with 17 outer-shell electrons. $Br$ belongs to the 4th row, so its outer shell is the 4th shell. 3) These 17 outer-shell electrons are distributed as follows: - The first two go to the $4s$ subshell. - The next 10 go to the $3d$ subshell. - The last 5 go to the $4p$ subshell. Therefore, the condensed electron configuration of $Br$ is $$Br: [Ar]4s^23d^{10}4p^5$$ 4) All the inner shells, the $4s$ subshell, and the $3d$ subshell are completely filled, so all of their electrons are paired. There are 5 electrons in subshell $4p$, and subshell $4p$ has 3 orbitals. According to Hund's rule, the first 3 electrons each occupy a different orbital, then the last 2 each occupy a one-electron orbital. So, that leaves 1 orbital with just 1 electron. Therefore, there is 1 unpaired electron.