Vanadium oxide cluster anions (V2O5)(n) VxOy- (n = 1-31; x = 0, 1; and x + y <= 5) with different oxygen deficiencies (Delta = 2y-1-5x = 0, +/-1, and 2) have been prepared by laser ablation and reacted to abstract hydrogen atoms from alkane molecules (n-butane) in a fast flow reactor. When the cluster size n is less than 25, the Delta = 1 series [(V2O5)(n)O- clusters] that can contain atomic oxygen radical anions (O center dot-) generally have much higher reactivity than the other four cluster series (Delta =-2,-1, 0, and 2), indicating that each atom counts in the hydrogen-atom abstraction (HAA) reactivity. Unexpectedly, all of the five cluster series have similar HAA reactivity when the cluster size is greater than 25. The critical dimension of vanadia particles separating the cluster behavior (each atom counts) from the bulk behavior (each atom contributes a little part) is thus about 1.6 nm (similar to V50O12s). The strong electron-phonon coupling of the vanadia particles has been proposed to create the O center dot- radicals (V5+ = O2-+ heat -> V4+-O center dot-) for the n > 25 clusters with Delta =-2,-1, 0, and 2. Such a mechanism is supported by a comparative study with the scandium system [(Sc2O3)(n)ScxOy- (n = 1-29; x = 0, 1; and x + y <= 4)] for which the A = 1 series [(Sc2O3)(n) O- clusters] always have much higher HAA reactivity than the other cluster series.