The following figure shows how the concentration of atomic and molecular gases changes as the altitude above the earth's surface increases. At ionospheric heights, the gases have thinned out considerably and atomic Oxygen, O, dominates molecular Oxygen, O 2 . In the ionosphere, protection is obtained when a neutral atom absorbs incoming radiation from the sun and becomes an ion when one of its electrons is freed. The photon gives up its energy in the collision and causes one of the electrons of the oxygen atom to be dislodged. This is the process by which ionization occurs.
Electrons have the same density.
The density of non-ionized, or neutral Oxygen atoms is around 500,000,000 per cm 3 or about 500 times as many in any given volume. We have used the heading image on this page to illustrate this point. The green dots would then represent the number of Nitrogen neutrals present in the same volume. There are 1000 of each. The ions in this volume would then be represented by the two yellow dots, a ratio of 500 to one.