Final answer:
The full electron configuration for a K- ion is 1s2 2s2 2p6 3s2 3p6 4s2 or [Kr]4s2 in noble gas shorthand.
Step-by-step explanation:
The electron configuration for a potassium (K) ion with a charge of -1 (K-) is determined by first writing the electron configuration for a neutral potassium atom and then adding one more electron because the ion has gained an extra electron.
A neutral potassium atom has the following electron configuration:
Since potassium has 19 electrons, the 19th electron goes into the 4s orbital. To form a K- ion, potassium gains one more electron which fills the next available orbital at 4p. Thus, the K- ion has 20 electrons and the full electron configuration:
Or using noble gas shorthand, the electron configuration can be written as [Kr]4s2.