The Classification of the Elements
In the periodic table the elements are classified according to Mendelejeff. The inert gases are placed in the last vertical column, and since they do not enter into combination with other elements their valency is zero. In the first group lithium, sodium, potassium, and so on, are to be found; they form compounds with those substances which can unite with one atom of hydrogen, so that they are called the monovalent elements. In the second group all those elements are to be found which are able to replace two atoms of hydrogen; the valencies of the elements in the next two groups are three and four respectively, after which the valency decreases. The elements of the first group are electropositive, they travel with the current during electrolysis and are deposited at the cathode. At the other end of the table the electronegative elements are found, that is they behave in an exactly opposite manner to the electropositive elements.