Video: Identifying the Electron Configuration of a Magnesium(II) Ion in a Set of Electron Configurations

Which of the following is the electron configuration for a magnesium(II) ion? [A] 1s² [B] 1s² 2s² 2p⁵ [C] 1s² 2s² 2p⁶ [D] 1s² 2s² 2p⁶ 3s¹ [E] 1s² 2s² 2p⁶ 3s¹ 3p²

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Video Transcript

Which of the following is the electron configuration for a magnesium(II) ion? A) 1s², B) 1s² 2s² 2p⁵, C) 1s² 2s² 2p⁶, D) 1s² 2s² 2p⁶ 3s¹, or E) 1s² 2s² 2p⁶ 3s¹ 3p².

The easiest way of working out the electron configuration for a magnesium(II) ion is to start with the electron configuration of a magnesium atom. The element magnesium is to be found in group two of the periodic table. It’s also in the third period. Magnesium has atomic number 12. This means that if we have a magnesium atom, there must be 12 protons in the nucleus and 12 electrons in the surrounding space. This is because atoms are electrically neutral. Protons and electrons have equal and opposite charge. So if we have 12 protons in a neutral particle, we must have 12 electrons. So for a magnesium atom, we have 12 electrons to work with.

If we lose two electrons to form the magnesium two plus ion, we must have 10 electrons in total. We start by placing two of those electrons in the 1s subshell, followed by two electrons in the 2s subshell and a further six electrons in the 2p subshell. So we’ve used up all the electrons we were given, exactly 10: two in the 1s subshell, two in the 2s subshell, and six in the 2p subshell. There’s only over one orbital in an s subshell. So we can only fit two electrons in those subshells. The 2p subshell can fit six electrons because it’s got three orbitals.

In the first electron shell, we only have the 1s subshell. So we have a maximum of two electrons there. And in the second electron shell, we have the 2s and 2p subshells, giving us a maximum of eight electrons. So we filled the first and second electron shells. And we don’t have to go further because we’ve run out of electrons.

So our final answer for the electron configuration for a magnesium(II) ion is 1s² 2s² 2p⁶.

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