Nápady 31+ Atomic System Of Units Zdarma . Atomic units, like si units, have a unit of mass, a unit of length, and so on. The atomic system is based on a unit of mass, the electron mass (approximately equal to 9.1093897 × 10 −31 kilograms), a unit of charge, the electron (approximately equal to 1.60217733 × 10 −19 coulombs), a unit of action, the planck (equal to planck's constant over 2 &invisibletimes; Atomic units (au) form a system of units convenient for atomic physics, electromagnetism, and quantum electrodynamics, especially when the focus is on the properties of electrons. There are two different kinds of atomic units, which one might name hartree atomic units and rydberg atomic units , which differ in the choice of the unit of mass and charge.