Chemistry: The Central Science (13th Edition)

Published by Prentice Hall
ISBN 10: 0321910419
ISBN 13: 978-0-32191-041-7

Chapter 6 - Electronic Structure of Atoms - Additional Exercises - Page 254: 6.102a


The condensed electron configuration of $Br$ is $$Br: [Ar]4s^23d^{10}4p^5$$ There is 1 unpaired electron.

Work Step by Step

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.
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