Periodicity and the Periodic Table
Mendeleev arranged the periodic table in order of Relative Atomic Mass (RAM.)
Elements with similar physical and chemical properties were placed into the same group. For example, Lithium (Li), Sodium (Na) and Potassium (K) are all reactive metals with low melting points.
He then swapped elements if he thought that they fit better into another group based on their physical and chemical properties e.g. Iodine and Tellurium. The RAM of Tellurium is higher than Iodine so the properties were not similar to other elements in the same group unless they were swapped over.
He left gaps for…