Video Transcript
Which of the following is the electron configuration for a sodium atom in the ground state. A) 1s², B) 1s² 2s² 2p⁵, C) 1s² 2s² 2p⁶, D) 1s² 2s² 2p⁶ 3s¹, or E) 1s² 2s² 2p⁶ 3s¹ 3p².
An atom in its ground state will have its electrons in the lowest possible energy levels. The 1s subshell has the lowest possible energy, followed by the 2s and 2p subshells. The pattern continues up until the 3p subshell where the next lowest energy subshell is actually the 4s subshell. And then, we get the 3d. You can use these diagonal arrows to help you remember the order. 1s 2s 2p and then 3s 3p and then 4s and then 3d and so on.
The highest occupied orbital in any of our candidates is in the 3p subshell. So we don’t need to worry too much about the particulars of the order beyond the 3p subshell. Instead, let’s go and have a look at a sodium atom.
The element sodium could be found in group one of the periodic table in period three. The atomic number of sodium is 11. So in an atom of sodium, in the nucleus, we have 11 protons. And in the space around the nucleus, we’ll have 11 electrons. An atom is neutral, so we must have as many protons as we have electrons. Therefore, for the electron configuration of a sodium atom in its ground state, we must have 11 electrons in total. We start with the lowest energy orbital in the 1s subshell, which contains a maximum of two electrons. After popping in two electrons, we have nine electrons left. In the 2s subshell, we can fit again a maximum of two electrons.
After popping in two electrons, our next stop is the 2p subshell. There are three orbitals in the 2p subshell. Each can take two electrons. So we have a maximum occupancy of six in the 2p subshell. With seven remaining electrons, we have enough to fill the 2p subshell. Leaving us with one electron, which we can place in the 3s subshell.
We’ve run out of electrons. And we’ve placed each electron in the lowest energy orbital possible. Therefore, we have the electron configuration of a sodium atom in its ground state. Of the five options given, the electron configuration for a sodium atom in the ground state is 1s² 2s² 2p⁶ 3s¹.
