order4dThe Order of Filling Orbitals. The auau principle explains how electrons fill low energy orbitals (closer to the nucleus) before they fill hher energy ones. Where there is aWe can solve a second order differential equation of the type: d 2 ydx 2 + P(x) dydx + Q(x)y = f(x). where P(x), Q(x) and f(x) are functions of x, by using: Undetermined Coefficients which only