The periodic law. Mendeleev’s periodic table
In 1869 D.I. Mendeleev arranged the elements by increasing atomic mass, noticed that their properties repeat periodically and stated the periodic law: the properties of simple substances and the forms and properties of compounds depend periodically on the atomic masses of the elements. In today’s wording this dependence is on the nuclear charge (the atomic number, that is, the electron configuration) rather than on the atomic mass. The table has 7 periods containing 2, 8, 8, 18, 18, 32 and 32 elements; every period except the first starts with an alkali metal and ends with a noble gas, and the vertical columns are groups, each divided into a main and a side subgroup. Moving left to right along a period the atomic radius decreases, metallic properties weaken, and non-metallic properties and electronegativity increase; down a main subgroup the atomic radius and metallic properties increase. The highest positive oxidation state of an element is usually equal to its group number, so the higher oxide of a group V element is E₂O₅ and that of group VI is EO₃; main-subgroup elements of groups IV–VII form volatile hydrogen compounds (formula EH₈₋ₙ, where n is the group number). Mendeleev left empty cells for undiscovered elements and predicted the properties of eka-aluminium (gallium, 1875), eka-boron (scandium, 1879) and eka-silicon (germanium, 1886); today 118 elements are known, and elements 113, 115, 117 and 118 were named nihonium, moscovium, tennessine and oganesson in 2016.
Table work: on a blank copy of the periodic table write the elements of periods 2 and 3 (Li–Ne, Na–Ar) and under each write the higher oxide and hydrogen compound formulas.