Nitrogen is a group 15 element. This means that it has three unpaired electrons in its valence shell p subshell. As a result, it tends to like to form three single bonds with other elements. Phosphorus and arsenic are also group 15 elements, so they behave in a similar fashion. Consequently, the formulas for the hydrides of phosphorus and arsenic are PH3 (phosphine) and AsH3 (arsine). Unlike ammonia (NH3), however, PH3 and AsH3 are both highly reactive and unstable in the atmosphere. Both will oxidize very easily from the oxygen in the air, and can only exist under inert conditions.