Be Beryllium II 9.3 Cd Cadmium II 112 Cs Cæsium I 133 Cr Cerium IV 92 D Didymium II 95 E Erbium II 112.6 Ir Iridium IV 198 La Lanthanum II 92 Li [3] Lithium I 7 Mo Molybdenum VI 96 Nb Niobium V 94 Os Osmium IV 199.2 Rh Rhodium IV 104.4 Rb Rubidium I 85.4 Ru Ruthenium IV 104.4 Ta Tantalum V 182 Tb Terbium Tl Thallium III 204 Th Thorium II 231.5 V Vanadium V 51.3 Y Yttrium II 61 Zr Zirconium III 89.6 All matter is made up of very small particles which are chemically indivisible and which are termed atoms, and the atom of each elementary substance differs essentially from that of every other. All the atoms of each element are alike, and chemical compounds are formed by the combination of unlike atoms. Hence the smallest particle of a compound consists of a group of atoms. This group, which can be divided by chemical but not by mechanical means, is termed a molecule. The smallest particle of an element in a free state is, however, not a single atom, but a group of atoms mechanically indivisible, or a molecule. This explains why elementary bodies act more energetically and enter more readily into combination at the moment of their liberation from a combination than when in the free state. When chemical changes occur, it is the molecules which react upon one another, and the change consists in the change of position of certain atoms contained in the groups. When an element is set free from a compound, the liberated join together to form molecules, unless some body is present with which the element can combine. By an atom we therefore understand the smallest portion of a chemical element which can enter into a chemical compound; by a molecule, the smallest portion of a simple compound body which can occur in the free state or which can take part in a chemical action.