Why isn't the scandium(2+) cation isoelectronic with potassium? I saw that I got this question wrong on a test, and was very confused.  Why aren't they isoelectronic?  $\ce{Sc^2+}$ with atomic number 21 should have 19 electrons since 21 - 2 = 19, which is the same as the number of electrons as Potassium with 19 protons and electrons.  Does it have something to do with being in different sublevels, one being s and the other d and all?
The neutral $\ce{Sc}$ atom has the electron configuration of $\ce{1s^2 2s^2 2p^6 3s^2 3p^6 3d^1 4s^2}$, and the $\ce{Sc^2+}$ ion has electron configuration $\ce{1s^2 2s^2 2p^6 3s^2 3p^6 3d^1}$.
The neutral potassium atom has the electron configuration of $\ce{1s^2 2s^2 2p^6 3s^2 3p^6 4s^1}$. While both $\ce{Sc^2+}$ and $\ce{K}$ have the same number of electrons, their electron configurations are different and therefore they are not isoelectronic to one another. (Isoelectronic implies same number of electrons as well as the same structure)