## Q. 3.5

Write the electron configurations for the following elements.

a. Strontium (atomic number = 38)
b. Lead (atomic number = 82)

## Verified Solution

a. The number of electrons in a strontium atom is 38. Remember that the atomic number gives the number of electrons (Section 3.2). We will need to fill subshells, in order of increasing energy, until 38 electrons have been accommodated.
$\quad\quad$The 1s, 2s, and 2p subshells fill first, accommodating a total of 10 electrons among them.
$\quad\quad\quad\quad 1s²2s²2p^{6}$ . . .
Next, according to Figures 3.10 and 3.11, the 3s subshell fills and then the 3p subshell.
$\quad\quad\quad\quad 1s²2s²2p^{6}\boxed{3s^{2}3p^{6}}$ . . .
We have accommodated 18 electrons at this point. We still need to add 20 more electrons to get our desired number of 38.
$\quad\quad$The 4s subshell fills next, followed by the 3d subshell, giving us 30 electrons at this point.
$\quad\quad\quad\quad 1s²2s²2p^{6}3s²3p^{6}\boxed{4s^{2}3d^{10}}$ . . .
Note that the maximum electron population for d subshells is 10 electrons.
$\quad\quad$Eight more electrons are needed, which are added to the next two higher subshells, the 4p and the 5s. The 4p subshell can accommodate 6 electrons, and the 5s can accommodate 2 electrons.
$\quad\quad\quad\quad 1s²2s²2p^{6}3s²3p^{6}4s²3d^{10}\boxed{4p^{6}5s^{2}}$
$\quad\quad$To double-check that we have the correct number of electrons, 38, we add the superscripts in our final electron configuration.
$\quad\quad\quad\quad$ 2 + 2 + 6 + 2 + 6 + 2 + 10 + 6 + 2 = 38
The sum of the superscripts in any electron configuration should add up to the atomic number if the configuration is for a neutral atom.
b. To write this configuration, we continue along the same lines as in part a, remembering that the maximum electron subshell populations are s = 2, p = 6, d = 10, and f = 14.

$\quad\quad$Lead, with an atomic number of 82, contains 82 electrons, which are added to subshells in the following order. (The line of numbers beneath the electron configuration is a running total of added electrons and is obtained by adding the superscripts up to that point. We stop when we have 82 electrons.)
$\underset{2 \ \ \ \ \ 4 \ \ \ \ \ 10 \ \ \ \ \ 12 \ \ \ \ \ 18 \ \ \ \ \ 20 \ \ \ \ \ 30 \ \ \ \ \ 36\ \ \ \ \ 38 \ \ \ \ \ 48 \ \ \ \ \ 54 \ \ \ \ \ 56\ \ \ \ \ 70\ \ \ \ \ \ 80\ \ \ \ \ 82}{1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^{10}4p^{6}5s^{2}4d^{10}5p^{6}6s^{2}4f^{14}5d^{10}6p^{2}}\\\quad\quad\quad\quad\quad \text{Running total of electrons added}$

$\quad\quad$Note in this electron configuration that the 6p subshell contains only 2 electrons, even though it can hold a maximum of 6. We put only 2 electrons in this subshell because that is sufficient to give 82 total electrons. If we had completely filled this subshell, we would have had 86 total electrons, which is too many.