Bohr model

as the electronic configuration is 1s^2 , 2s^2 , 2p^6, 3s^2, 3p^6, 4s^1

In the Bohr model electron configuration notation, electrons are assigned to energy levels with no distinctions made for angular momentum or spin. The notation used for energy levels $n =$ 1, 2, 3, 4, 5, 6 ... is a capital letter K, L, M, N, O, P ... (historically based on x-ray spectroscopy notation). The maximum number of electrons allowed in each energy level is shown here in parentheses: K(2), L(8), M(8), N(18), O(18), P(36), .... So, for potassium $\ce K$ with 19 electrons, the electron configuration for the ground state is formed by filling up energy levels from the bottom up: 19 = 2 + 8 + 8 + 1 $\Rightarrow$ $\boxed{\text{K(2)L(8)M(8)N(1)} }$ .