Gold is a stable heavy metal that is easily formed into uniform and thin sheets of measurable property. Alpha particles are relatively massive (compared to electrons) and so deflection is less dramatic and more easily measured. This is the property that prompted Rutherford to use alpha particles and gold.
The particles that are positively charged were fired at the gold foil and when it reached that goldatoms’ nuclei, which are also positively charged, the particles were repelled back
