When alpha particles are sent through a thin metal foil, most of them go straight through the foil because

Conclusion taken from Rutherford's model:

• Atom has most space as empty.
• A very small mass is concentrated in a small area called a nucleus. The mass of an atom is concentrated at the central nucleus and the remaining part is empty. 
• Therefore, the positively charged alpha particles when sent through metal foil go straight through the foil.