Nonmetals. The halogens

Nonmetals stand at the ends of the short and long periods, in the main subgroups; except hydrogen and helium they are all p-elements. Their atoms tend to accept electrons, so in reactions they are often oxidizing agents; this property grows from left to right in a period and weakens down a subgroup. The nonmetal that accepts electrons most strongly is fluorine, followed by oxygen and chlorine. Their highest oxides are acidic (group IV RO₂, group V R₂O₅, group VI RO₃, group VII R₂O₇), and their volatile hydrogen compounds have the formulas RH₄, RH₃, H₂R, HR. Under ordinary conditions H₂, N₂, O₂, F₂, Cl₂ are gases, bromine is a liquid, and sulfur, phosphorus, carbon and iodine are solids. The oxygen subgroup is called chalcogens, the fluorine subgroup halogens and the helium subgroup noble gases. By mass the Earth’s crust holds about 47 % oxygen and 28 % silicon.