ns2np3

ns2np3 is the shorthand used in atomic electron configuration to describe the outermost valence shell of certain main-group elements. It denotes two electrons in the s subshell and three electrons in the p subshell of the same principal quantum number n, giving a total of five valence electrons. This ns^2 np^3 configuration is characteristic of the Group 15 elements, also known as the pnictogens, across periods as n increases: nitrogen (2s2 2p3), phosphorus (3s2 3p3), arsenic, antimony, bismuth, and the heavier congeners in period 7.

Chemically, five valence electrons predispose these elements to form covalent bonds and to exhibit multiple oxidation states, commonly -3 in hydrides and +3 or +5 in oxides and related compounds. In nitrogen compounds, for example, ammonia (NH3) features three N–H bonds and a lone pair, reflecting sp3 hybridization of the ns2np3 valence. Phosphorus forms compounds such as PCl3 and PCl5 with oxidation states +3 and +5. Heavier pnictogens also display +3 and +5 chemistry, as seen in various oxides and halides.

Notes and limitations: the ns2np3 designation is a simplified model of the outer shell and is most accurate for a qualitative view of bonding and periodic trends. In heavier elements, d-orbital participation and relativistic effects can modify the straightforward picture, so actual electronic structures may deviate somewhat from the simple ns^2 np^3 picture. The notation remains a convenient way to discuss valence electrons and chemical behavior in the pnictogen family.