Chapter 5 - Electron Configurations and the Periodic Table - Questions for Review and Thought - Topical Questions - Page 240c: 70c

$1s^22s^22p^6$

From the given information, we see that Ca 2+ and K+ are the two isoelectric species present, since they have the same number of electrons. We now write the electron configurations, using the information that we learned over the course of the chapter. Recall, we start at 1s and move up: $1s^22s^22p^6$