Momentum accommodation is a key factor governing the transport of particles in gases from electric mobility and Brownian diffusion to thermophoresis. This paper explores the relationship between momentum accommodation of nanoparticles in dilute gases and surface adsorption. We demonstrate that the momentum accommodation factor is fundamentally equal to the probability of surface adsorption. Molecular dynamics simulations show that surface adsorption is the key mechanism behind the diffuse scattering model, and that upon gas-particle collision the immediate reflection dynamics and surface adsorption events are governed by the kinetic energy distribution of the rebounding gas molecules. This distribution determines the transition of the dominant mode of molecular scattering off a particle surface from diffuse to specular elastic reflection as the particle approaches the realm of molecular size. The kinetics and equilibrium of physisorption are examined to shed light on the effect of the lifetime of surface adsorbates on momentum transfer. A statistical treatment is proposed for the adsorption and hence the gas-nanoparticle momentum accommodation coefficient. The validity of the theoretical treatment is examined by comparing its prediction against experimental mobility data of silver nanoparticles.