Thomson observed two separate patches of light on the photographic plate (see image), which suggested two different parabolas of deflection. Aston subsequently discovered multiple stable isotopes for numerous elements using a mass spectrograph.
Primordial nuclides include 32 nuclides with very long half-lives (over 100 million years) and 253 that are formally considered as "stable nuclides", because they have not been observed to decay.
In most cases, for obvious reasons, if an element has stable isotopes, those isotopes predominate in the elemental abundance found on Earth and in the Solar System.
The neutron number has large effects on nuclear properties, but its effect on chemical properties is negligible for most elements.
Even in the case of the lightest elements where the ratio of neutron number to atomic number varies the most between isotopes it usually has only a small effect, although it does matter in some circumstances (for hydrogen, the lightest element, the isotope effect is large enough to strongly affect biology).
radioactive elements) between uranium and lead, although the periodic table only allowed for 11 elements from uranium to lead.