The substances with which the chemist has to deal admit of classification into elements and compounds. Of the former about eighty may be regarded as well characterized, although many more have been described.

Elements.—The following table gives the names, symbols and atomic weights of the perfectly characterized elements:—

International Atomic Weights, 1910.

Name. Symbol. Atomic
Weights.
O=16.		 Name. Symbol. Atomic
Weights.
O=16.
Aluminium Al 27. 1Mercury Hg 200. 0
Antimony Sb 120. 2Molybdenum Mo 96. 0
Argon A 39. 9Neodymium Nd 144. 3
Arsenic As 74. 96Neon Ne 20
Barium Ba 137. 37Nickel Ni 58. 68
Beryllium or Be } 9. 1Nitrogen N 14. 01
Glucinum Gl } Osmium Os 190. 9
Bismuth Bi 208. 0Oxygen O 16. 00
Boron B 11. 0Palladium Pd 106. 7
Bromine Br 79. 92Phosphorus P 31. 0
Cadmium Cd 112. 40Platinum Pt 195. 0
Caesium Cs 132. 81Potassium K 39. 10
Calcium Ca 40. 09Praseodymium Pr 140. 6
Carbon C 12. 0Radium Ra 226. 4
Cerium Ce 140. 25Rhodium Rh 102. 9
Chlorine Cl 35. 46Rubidium Rb 85. 45
Chromium Cr 52. 0Ruthenium Ru 101. 7
Cobalt Co 58. 97Samarium Sa 150. 4
Columbium Cb } 93. 5Scandium Sc 44. 1
or Niobium Nb } Selenium Se 79. 2
Copper Cu 63. 57Silicon Si 28. 3
Dysprosium Dy 162. 5Silver Ag 107. 88
Erbium Er 167. 4Sodium Na 23. 0
Europium Eu 152. 0Strontium Sr 87. 62
Fluorine F 19. 0Sulphur S 32. 07
Gadolinium Gd 157. 3Tantalum Ta 181. 0
Gallium Ga 69. 9Tellurium Te 127. 5
Germanium Ge 72. 5Terbium Tb 159. 2
Gold Au 197. 2Thallium Tl 204. 0
Helium He 4. 0Thorium Th 232. 42
Hydrogen H 1. 008Thulium Tm 168. 5
Indium In 114. 8Tin Sn 119. 0
Iodine I 126. 92Titanium Ti 48. 1
Iridium Ir 193. 1Tungsten W 184. 0
Iron Fe 55. 85Uranium U 238. 5
Krypton Kr 83. 0Vanadium V 51. 2
Lanthanum La 139. 0Xenon Xe 130. 7
Lead Pb 207. 10Ytterbium
Lithium Li 7. 00 (Neoytterbium) Yb 172
Lutecium Lu 174 Yttrium Y 89. 0
Magnesium Mg 24. 32Zinc Zn 65. 37
Manganese Mn 54. 93Zirconium Zr 90. 6

The elements are usually divided into two classes, the metallic and the non-metallic elements; the following are classed as non-metals, and the remainder as metals:—

HydrogenOxygenBoronNeon
ChlorineSulphurCarbonKrypton
BromineSeleniumSiliconXenon
IodineTelluriumPhosphoruselium
FluorineNitrogenArgon

Of these hydrogen, chlorine, fluorine, oxygen, nitrogen, argon, neon, krypton, xenon and helium are gases, bromine is a liquid, and the remainder are solids. All the metals are solids at ordinary temperatures with the exception of mercury, which is liquid. The metals are mostly bodies of high specific gravity; they exhibit, when polished, a peculiar brilliancy or metallic lustre, and they are good conductors of heat and electricity; the non-metals, on the other hand, are mostly bodies of low specific gravity, and bad conductors of heat and electricity, and do not exhibit metallic lustre. The non-metallic elements are also sometimes termed metalloids, but this appellation, which signifies metal-like substances (Gr. εἰδος, like), strictly belongs to certain elements which do not possess the properties of the true metals, although they more closely resemble them than the non-metals in many respects; thus, selenium and tellurium, which are closely allied to sulphur in their chemical properties, although bad conductors of heat and electricity, exhibit metallic lustre and have relatively high specific gravities. But when the properties of the elements are carefully contrasted together it is found that no strict line of demarcation can be drawn dividing them into two classes; and if they are arranged in a series, those which are most closely allied in properties being placed next to each other, it is observed that there is a more or less regular alteration in properties from term to term in the series.

When binary compounds, or compounds of two elements, are decomposed by an electric current, the two elements make their appearance at opposite poles. Those elements which are disengaged at the negative pole are termed electro-positive, or positive, or basylous elements, whilst those disengaged at the positive pole are termed electro-negative, or negative, or chlorous elements. But the difference between these two classes of elements is one of degree only, and they gradually merge into each other; moreover the electric relations of elements are not absolute, but vary according to the state of combination in which they exist, so that it is just as impossible to divide the elements into two classes according to this property as it is to separate them into two distinct classes of metals and non-metals. The following, however, are negative towards the remaining elements which are more or less positive:—Fluorine, chlorine, bromine, iodine, oxygen, sulphur, selenium, tellurium.

The metals may be arranged in a series according to their power of displacing one another in salt solutions, thus Cs, Rb, K, Na, Mg, Al, Mn, Zn, Cd, Tl, Fe, Co, Ni, Sn, Pb, (H), Sb, Bi, As, Cu, Hg, Ag, Pd, Pt, Au.

Elements which readily enter into reaction with each other, and which develop a large amount of heat on combination, are said to have a powerful affinity for each other. The tendency of positive elements to unite with positive elements, or of negative elements to unite with negative elements, is much less than that of positive elements to unite with negative elements, and the greater the difference in properties between two elements the more powerful is their affinity for each other. Thus, the affinity of hydrogen and oxygen for each other is extremely powerful, much heat being developed by the combination of these two elements; when binary compounds of oxygen are decomposed by the electric current, the oxygen invariably appears at the positive pole, being negative to all other elements, but the hydrogen of hydrogen compounds is always disengaged at the negative pole. Hydrogen and oxygen are, therefore, of very opposite natures, and this is well illustrated by the circumstance that oxygen combines, with very few exceptions, with all the remaining elements, whilst compounds of only a limited number with hydrogen have been obtained.