Nitrogen, phosphorous, sulfur, and oxygen are all nonmetals. However, oxygen is the most reactive of the four. This is because oxygen has the highest electronegativity of the four. Electronegativity is a measure of an atom's ability to attract electrons. The more electronegative an atom is, the more it attracts electrons. Oxygen's high electronegativity means that it is very good at attracting electrons from other atoms. This makes it very reactive.