When atoms are bombarded with $\mathrm{\alpha }$-particles, only a few in a million of the $\mathrm{\alpha }$-particles, suffer deflections while others pass through undeflected. This is because :

the size of nucleus do not have any effect on the $\mathrm{\alpha }$-particles

the force of repulsion on the fast moving $\mathrm{\alpha }$-particle is small

the force of attraction on the $\mathrm{\alpha }$-particles by the oppositely charged electrons is not sufficient

the nucleus occupies much smaller volume compared to the volume of the atom