Neon: discovery, 54 electrons

Ion chemistry is a product of the 20th century. J J Thomson discovered the electron in 1897 and identified it as a constituent of all matter. Free positive ions (as distinct from ions deduced to exist in solids or electrolytes) were first produced by Thomson just before the turn of the century. He produced beams of light ions, and measured their mass-to-charge ratios, in the early 1900s, culminating in the discovery of two isotopes of neon in 1912 [1]. This year also marked Thomson s discovery of which turns out to be the... 

Thomson was engaged in the measurement of the e/m of positive rays by the same general method as he used f or the electron when, in 1913, he discovered that neon consisted of two different kinds of atoms one having a mass of 20, the other having a mass of 22. These different atoms of the same element are called isotopes, meaning same place (that is, in the periodic table). Since this discovery that an element may contain atoms of different mass, the isotopic constitution of all the elements has been determined. Moreover, as is well known, in recent years many artificial isotopes have been synthesized by the high-energy techniques of physics. 

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