- An atom or positive ion N, P, As, S, or Se is not treated as stereogenic if it has (a) a terminal H atom neighbor or (b) at least two terminal neighbors, −XH
m
and − XH
n
, (n + m > 0) connected by any kind of bond, where X is O, S, Se, Te, or N. Phosphines and arsines are always treated as stereogenic even with H atom neighbors.