This document provides information about allotropy and oxoacids of elements in groups 16, 17 and 18 of the periodic table. It discusses the allotropes of oxygen, sulfur, selenium and tellurium. It also describes various oxoacids of sulfur like sulphurous acid, sulphuric acid and others. The document further discusses oxoacids of halogens and provides structures of oxoacids of chlorine. It then covers various topics related to oxygen and its compounds including dioxygen, ozone and simple oxides.