Monatomic alloying of atomic clusters emerges as a promising means for efficient catalyst development with a yet unclear mechanism. Here we show that such monatomic cluster alloying takes the advantages of both atomic undercoordination and atomic heterocoordination that enhance each other in the local quantum entrapment and polarization. Atomic undercoordination shortens the Sc–Me bond by up to 30% and reduces the atomic cohesive energy by 25% (Me = Ag, Au, Ca, Cd, Cu, Ni, Ir, Ti, Rh, Pd, Fe, Pt, Y, Zn), resulting in local energy density gain and charge polarization. Monatomic alloying enhanced the effect of undercoordination on the quantum entrapment and local polarization, which is fundamentally significant in modulating the catalytic ability of metal-doped Sc clusters.