Why was the plum pudding model for atoms doubted?
Thomson’s raisin bun model of the atom, where electrons are embedded in a blob of massless positive charge, was invalidated by the gold foil experiment (http://en.wikipedia.org/wiki/Geiger-Marsden_experiment). Alpha particles (i.e., helium nuclei: two protons and two neutrons) fired at gold foil were expected to pass through unobstructed under Thomson’s model, but occasionally deflected back at an extremely sharp angle — some essentially doubled back at a 180 degree angle. This suggested that the positive charge of the gold foil’s atoms was concentrated in a tiny mass at the centre of the atom, producing a large repulsive electric force on the (rare) occasions that the positively-charged alpha particles closely approached the proton-containing nuclei of the foil’s atoms.
