Why were alpha particles deflected by the Rutherford's gold -foil experiment?
Due to the positively charged nucleus of the gold atoms.
Alpha particles are are positively charges particles that are made up of 2 protons, 2 neutrons and zero electrons. Due to the fact that protons have a +1 charge and neutrons hold no charge, this would give the particle a +2 charge over all.
Originally Rutherford thought that the particles would fly straight through the foil. However, he found that the particles path would be shifted or deflected when passing through the foil. This is due to the fact that like charges repel each other.
As the positively charged alpha particle would fly through the foil it would come in proximity with the positively charge nucleus of the atom. This in turn either deflected the particle or adjusted its path.