Mendeleev's table

The Russian chemist Dmitri Mendeleev published his periodic table of the elements in 1869. This table contains all the known chemical elements, arranged in order of the number of protons: their atomic number. The reason why it is called the periodic table is that elements with similar properties occur in it at regular intervals, arranged in columns.

Elements in the same column have similar chemical properties. We also find several families of elements:

• The true metals consisting of the alkaline and alkaline earth metals;
• the transition metals;
• the metalloids;
• the non-metals;
• the halogens;
• the noble gases;
• the lanthanides;
• the actinides;
• the transuranic elements.

Mendeleev's discovery of the periodic table

The first version of the periodic table, the one published in 1869, was written in a single day. This day has been retraced in detail and we know that on the morning of 17 February 1869 (1 March in our calendar), Mendeleev received two letters from A. I. Khodnev about a visit to a dairy. Mendeleev probably received them at breakfast, as suggested by a cup stain on one of them. Mendeleev was trying to arrange the table of elements, but without success. So he went off to inspect the dairy, putting off his classifying problem till later on. In a second paper dated that same day, Mendeleev made two other unsuccessful attempts at a classification. Then he received a visit from his friend A. A. Inostrantsev, who later remembered having found Mendeleev in a thoughtful mood, explaining that he had an idea for classification in his head but that he could not find a way of putting it on paper.

After this visit, Mendeleev wrote a list of the exact atomic weights in the margin of his book "Principles of Chemistry". From this list, he compiled separate cards of the 63 known elements, with their atomic weight and their main chemical properties. Then for several hours he tried to arrange the cards. He finally copied the arrangement on to a sheet of paper on which can be seen many modifications. Mendeleev went for a nap; on waking up, he wrote out the classification in one go, as it was published in his first communication. Legend has it that he saw his classification in a dream.

In his lifetime, Mendeleev had the pleasure of knowing that three of the elements whose existence he had predicted were discovered (gallium, scandium and germanium), and that the physical and chemical properties that he had predicted for these elements and some of their compounds were correct. In some cases, he had even predicted how the elements would be discovered, and this was borne out.