The Swedish chemist Berzclius (1779-1848) introduced the modern symbols for the elements when he published a list of "atomic weights" in 1818. Those elements which, on account of their case of extraction from their ores had been known from prehistoric times, were given symbols derived from their Latin names.
The symbols began as a simple shorthand for referring to the names of the elements, but rapidly acquired a quantitative significance. Pb began meaning "some lead", but it rapidly changcd and camc to mean cither "one atom of lead" or "one gram atomic weight of lead", i. e. 207 g of lead.
Some elements can exhibit allotropy; that is to say, they can exist in two different forms in the same physical state. Taking examples from among gaseous elements, ozone and molecular oxygen both contain only oxygen atoms, yet, the reactions of ozone are completely different from those of oxygen. It will immediately oxidize rubber and metallic silver, whereas oxygen will do neither. Evidence from gas-volume determinations shows that three atoms of oxygen are associated in ozone, whereas there are only two atoms in molecular oxygen. The atoms of all common gaseous elements exist in pairs except the inert gases. To avoid confusion, molecular gases are written 02, N2, Cl2 and ozone Oj.
Some elements also exhibit allotropy; for example, white phosphorus catches fire in air at room temperature while red phosphorus is stable in air at 240°C. But since there is no way of discovering how many atoms arc combined together in any solid, the question is sidestepped, and all solid elements including allotropcs arc given symbols; thus, both white and red phosphorus are P rather than Px.
