The neon ions ended on a photographic plate, which was darkened at the point of landing. There were two regions of darkening, because there were neon ions of two different masses that curved in two different degrees and ended in two different places. Thomson showed, from the amount of curving, that there was a neon isotope with an atomic weight of 20 and one with an atomic weight of 22—²⁰Ne and ²²Ne.
What’s more, from the intensity of darkening, it could be seen that ordinary neon was made up of atoms that were roughly 90% ²⁰Ne and 10% ²²Ne. The overall atomic weight of neon, 20.2, was the average atomic weight of these 2 isotopes.
Thomson’s instrument was the first one capable of separating isotopes and such instruments came to be called “mass spectrometers”. The first to use the name was the English physicist Francis William Aston (1877-1945), who built the first efficient instrument of this type in 1919.
He used it to study as many elements as he could. He and those who followed him located many isotopes and determined the frequency of their occurrence with considerable precision. It turned out, for instance, that neon is actually 90.9% ²⁰Ne, and 8.8% ²²Ne. Very small quantities of still a third isotope, ²¹Ne, are also present, making up 0.3%.
As for ordinary lead in nonradioactive rocks, it is made up of 23.6% ²⁰⁶Pb, 22.6% ²⁰⁷Pb, and 52.3% ²⁰⁸Pb. There is still a fourth isotope, ²⁰⁴Pb, which makes up the remaining 1.5% and which is not the product of any radioactive series at all.
The isotopes always have atomic weights that are close to, but not quite, whole numbers. Any atomic weight of an element that departs appreciably from an integer does so only because it is an average of different isotopes. For instance, the atomic weight of chlorine (chemical symbol Cl) is 35.5, but this is because it is made up of a mixture of 2 isotopes. About one quarter of chlorine’s atoms are ³⁷Cl and about three-quarters are ³⁵Cl.
Francis W. Aston
Mass spectrograph as used by Thomson and Aston to measure the atomic weight of neon.