When we calculate coulomb's force between the Th atom and the alpha particle, why do we not consider the attractive forces on the alpha particle due to the electrons of the Th atom? 

This is due to size of atom and size of nucleus. Size of atom is around 10^-12 m .

Nucleus is concentrated at centre of atom and its size is 10^-15 m .

 

Hence when α particle is at a distance 9 × 10^-15 m from centre of nucleus/atom ,

 

it is very close to +ve charge nucleus .  Hence Coulombic repulsive foce between nucleus and α particle

is much greater than attaractive force due to electron