what do you mean by catenation?

Catenation is Connection of links or union of parts, as in a chain; a regular or connected series. Catenation in nitrogen is limited to two or three atoms . Hydrazine , NH₂ - NH₂ , the  simplest catenated N-N compound is unstable with respect to disproportionation  to elements . The higher homologues become increaseingly unstable . The P-P compounds are comparatively more  stable . In As , Sb and Bi , the property of catenation is still less and decrease in the order P > N > As >> Sb >>> Bi .