Which of the following is the ground-state electron configuration of a neutral silicon, Si, atom? A) 1s² 2s² 2p², B) 1s² 2s² 2p⁶ 3s², C) 1s² 2s² 2p⁶ 3s² 3p², D) 1s² 2s² 2p⁶ 3s² 3p⁶ 4s², or E) 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 4p².
An atom in its ground state will have all its electrons in the lowest energy orbitals available. In electron configurations, we use a notation that shows the occupancy of each subshell. The number indicates the shell number and the letter s, p, d, or f, and so on is the type of orbital in that subshell. The number to the top right indicates how many electrons there are in total in that subshell. The question is asking us for the electron configuration of a silicon atom in its ground state. So let’s have a think about what we know about silicon.
We can find the element silicon in Group 14 of the periodic table, sometimes called Group four. And we can see from the periodic table that the atomic number of silicon is 14. Therefore, in an atom of silicon, there will be 14 protons in the nucleus, since the atomic number is simply the number of protons per nucleus for atoms or ions of that element. The question tells us that this atom is neutral, meaning that it has no overall charge. Although this is generally accepted to be part of the meaning of atom, it’s nice to have a reminder. To be neutral, we need as many electrons as we have protons. So, we need 14 electrons to balance out the charge of our 14 protons.
Now, we can start constructing the electron configuration for our silicon atom. The lowest energy subshell is the 1s subshell. s-type subshells can only fit two electrons. So, our first two electrons go into the 1s subshell. The next lowest energy subshell is the 2s subshell. So, that’s where we put our next two electrons. There’s enough space in the second electron shell to fit p-type orbitals. So, we have a 2p subshell as well in the second electron shell. p-type subshells fit a maximum of six electrons. So, we can put the next six of our remaining electrons in there.
And we now move to the third electron shell and the 3s subshell. Just like the 2s and the 1s subshells, we can fit two electrons in the 3s subshell. And finally, we have the 3p subshell. However, we’ve only got two electrons left. So, the 3p subshell isn’t going to be filled completely. We put our last two electrons in. And since we’re out of electrons, we’ve completed the ground-state electron configuration of a silicon atom. This matches the third answer.
If we’d had more electrons, we might have had to start filling the 4s subshell before filling the 3d subshell. If you arrange the subshells in the way I’ve done in this diagram, you can use diagonal arrows to tell you which one will be next in energy order. But in this case, it wasn’t necessary. A potential shortcut for this question would be to realize that silicon is in period three of the periodic table. So, we expect its valence electrons to be present in the third electron shell. Also, we expect atoms of silicon to have a valency of four because they’re in Group 14.
So, one of the ways you could have figured out this question was by looking at the electron configurations and finding the one with four electrons in the third electron shell. Whichever way you do it, you should have ended up with a ground-state electron configuration of a neutral silicon atom of 1s² 2s² 2p⁶ 3s² 3p².