Aufbau's principal of the arrangement of electronic configuration states that, electrons fill orbitals of lower energy first before the higher energy. 1s2 2s2 2p6 3s2 3p6 4s2 3d10...
This arrangement explains the Aufbau's principal, hence, or therefore, the 1s2 2s2 2p6 3s2 3p6 4s1 is the right answer for the representation of the electron configuration for potassium (K).
