Non metallic elements that occur in a combined state are extracted from their compounds by oxidation. Highly electronegative non metals such as chlorine and fluorine are obtained by electrolytic oxidation. They cannot be obtained by ordinary chemical oxidation because they themselves are the strongest reducing agents. Non metals nitrogen, oxygen, and noble gases occur in a free state in air and are easily obtained by fractional distillation of liquid air. Carbon is extracted in the native state. Sulphur and phosphorus are extracted from native ore or are obtained by the chemical oxidation of their compounds occurring in nature.