# Video: Identifying the Element Whose Atoms Have a Given Number of Electrons in Their Outer s Orbital in a Set of Elements on an Abbreviated Periodic Table

Which of the letters corresponds to the element whose atoms have only one electron in their outer s orbital, in their ground state?

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### Video Transcript

Which of the letters corresponds to the element whose atoms have only one electron in their outer s orbital, in their ground state?

And here, we’re given an abbreviated periodic table with most of the elements missing, but some filled in. The ground state is simply the lowest energy electron configuration for an atom. If we fill orbitals with electrons and increasing energy as usual, we will have arrived at the ground state electron configuration for that atom. When I say as usual, I mean according to a number of rules such as the Aufbau principle that can be summarised using this handy diagram for filling orbitals.

Magnesium, for example, has 12 electrons. Two electrons convert into the first s orbital. The one s orbital is followed by the two s orbital, which can also fit two electrons. Next, we’ll fill the two p orbitals, which can fit a total of six electrons. The next highest energy orbital is the three s orbital, which can also fit two electrons. And now, we’ve taken care of all of magnesium’s 12 electrons. So magnesium’s ground state electron configuration is one s two, two s two, two p six, and three s two. The last two electrons in the three s orbital make up the electrons that are in the outer or valence shell of magnesium.

In this question, we want to figure out which element will only have one electron in the outer s orbital. If we look at our diagram, we’ll notice that, of the orbitals in the shell — so, for example, the two s and the two p orbital are in the second shell — the s orbital gets filled first. This is because the s orbital is lower in energy. Since the s orbital gets filled first for a shell, if there is only one electron in the outer s orbital, that means that there’s only one electron in the outer shell.

So what this question is really asking is which letter corresponds to an element that only has one electron in its outer shell. Elements in group one of the periodic table all only have one electron in their outer shell. So the element that we’re looking for must be letter A, which corresponds to the element sodium. So sodium would only have one electron in their outer s orbital in the ground state.