When cobalt is heated, it reacts with halogens and other nonmetals. Examples of these nonmetals are boron, carbon, phosphorus, arsenic, antimony, and sulfur. In its compound, cobalt shows all oxidation states from I- IV. The most common of these states being II and III. The highest oxidation level, IV is found in cesium hexafluorocobaltate and very few other compounds.  

Reaction of cobalt with air and water- In general cobalt doesn’t react with air until it is heated and the oxide Co3O4 is created. When cobalt is heated above 900°C, cobalt(II) oxide, CoO is formed. Once again, water has little effect on cobalt until the metal is heated and steam creates CoO.
3Co(s) + 4O₂(g) --> 2Co₃O₄(s)
2Co(s) + O₂(g) --> 2CoO(s)

Reaction of cobalt with the halogens- Cobalt(II) bromide, CoBr2 is created when cobalt metal and bromine react with one another, the final reaction gives off a green color. Corresponding chloride and iodide can also be produced in the same way with the final reaction of the chloride being blue, and iodide black.
Co(s) + Br₂(l) --> CoBr₂(s)
Co(s) + Cl₂(g)--> CoCl₂(s)
Co(s) + I₂(s) --->CoI₂(s)

Reaction of cobalt with acids- When cobalt is introduced to sulphuric acid, the metal will slowly dissolve.
Co(s) + H₂SO₄(aq) --> Co²⁺(aq) + SO₄^2-(aq) + H₂(g)