Elements have atoms that are essentially all the same. Elements may occur as allotropes. Example: Graphite and diamond are both allotropes of carbon.
Chemical elements have characteristics that can be measured. The radius of atoms as you move to the right on a row of the periodic table gets smaller.
Chemical elements have a fixed density, fixed melting and boiling points, fixed latent heats and fixed specific heats, if they are in the same allotropic form.
Once people could look at pure samples of oxygen, phosphorus and so on, they were most of the way to accepting that atoms, once mere theories, really existed.
Elements show patterns in reactivity, and a displacement reaction provides clear evidence of relative reactivity when two elements are compared with each other.
By John Dalton's time, many different chemists in western and northern Europe were beginning to discover and prepare pure samples of the different elements.
Some elements can exist in one stable form or allotrope, with varying properties. Elements with allotropes include carbon, phosphorus, oxygen, sulfur and tin.
A small number of elements are able to form ions with more than one charge: examples include iron, copper and mercury. The properties of the ions are different.
In 1894 Lord Rayleigh and William Ramsay discovered argon by spectroscopic analysis of the gas left over after nitrogen and oxygen are removed from air.
Stanislao Cannizzaro popularized the idea that molecules of elements need not be single atoms, explaining a number of puzzles about gases, up until then.
William Prout's lasting fame comes from his anonymous suggestion in 1815, that the atomic weights of the elements were all multiples of that of hydrogen.
In effect, William Prout argued, in what was later called 'Prout's hypothesis', that all atoms are made up of clusters of hydrogen atoms in varying numbers.
In 1789, Antoine Lavoisier described conservation of mass in chemical reactions, listing 31 substances believed to be elements (eight were compounds).
In 1811, Bernard Courtois discovered the element iodine, while making potassium nitrate from ash derived from seaweed, as part of France's war effort.
This file is http://members.ozemail.com.au/~macinnis/scifun/splatselements.htm, first created on January 25, 2006. Last recorded revision (well I get lazy and forget sometimes!) was on January 25, 2006.