This form for the ionization rate formula_14 emphasizes that the characteristic electric field needed for ionization formula_15 is proportional to the ratio of the ionization energy formula_16 to the characteristic size of the electron's orbital formula_17. Thus, atoms with low ionization energy (such as alkali metals) with electrons occupying orbitals with high principal quantum number formula_18 (i.e. far down the periodic table) ionize most easily under a DC field. Furthermore, for a Hydrogenic atom, the scaling of this characteristic ionization field goes as formula_19, where formula_20 is the nuclear charge. This scaling arises because the ionization energy scales as formula_21 and the orbital radius as formula_22. More accurate and general formulas for the tunneling from Hydrogen orbitals can also be obtained.