Mendeleev arranged the elements on the right side of his...

Mendeleev arranged the elements on the right side of his periodic table according to the formulas of the binary compounds they form with hydrogen, and he used those formulas as column labels. Which groups of the modern periodic table were labeled "HR," "H $_{2} \mathrm{R}, "$ and " $\mathrm{H}_{3} \mathrm{R}^{n}$ in Mendeleev's table, where "R" represented one of the elements in the group?

Key Concepts

Pnictogens

The designation ‘H?R?’ in Mendeleev’s table refers to the elements we now group as the pnictogens or nitrogen family (Group 15). These elements, which include nitrogen and phosphorus, typically form hydrides with three hydrogen atoms (like NH?), reflecting their trivalent bonding character and related chemical behavior.

Halogens

In Mendeleev’s table, the column labeled ‘HR’ corresponds to what we now call the halogens (Group 17). These elements typically form compounds with one hydrogen atom (as in HF, HCl, etc.) and are known for their high electronegativity, diatomic nature, and vigorous chemical reactivity.

Chalcogens

The column labeled ‘H?R’ in Mendeleev’s classification corresponds to the modern chalcogens (Group 16). Elements in this group combine with two hydrogen atoms (for example, in H?O and H?S) and share chemical properties such as similar oxidation states and the tendency to form oxides and other compounds.

Periodic Table Grouping

The modern periodic table is organized into groups (columns) that gather elements with similar chemical properties and electron configurations. This grouping system allows for the prediction of trends in reactivity, bonding, and other chemical behaviors.

Binary Hydride Formulas in Early Classification

Before the modern understanding of atomic structure, early chemists like Mendeleev used the formulas of binary compounds (hydrides) to classify elements. By observing the ratios in which elements combined with hydrogen (such as HR, H?R, H?R?), they could infer valencies and chemical similarities among the elements.

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