out over the atom's entire volume. According to Coulomb's Law, the less concentrated a sphere of electric charge is, the weaker its electric field at its surface will be. As a worked example, consider an alpha particle passing along the edge of a gold atom, where it will experience the electric field at its strongest and thus experience the maximum deflection θ. Since the electrons are very light compared to the alpha particle, their influence can be neglected, so the atom can be seen as a heavy sphere of positive charge. Qn = positive charge of gold atom = = Qα = charge of alpha particle = = r = radius of a gold atom = vα = velocity of alpha particle = mα = mass of alpha particle = k = the Coulomb constant = Using classical physics, the alpha particle's lateral change in momentum Δp can be approximated using the impulse of force relationship and the Coulomb force expression: The above calculation is but an approximation of what happens when an alpha particle comes near a Thomson atom, but it is clear that the deflection at most will be in the order of a small fraction of a degree. If the alpha particle were to pass through a gold foil about 4 micrometres thick (2,410 atoms) and experience maximal deflection in the same direction (unlikely), it would still be a small deflection. The outcome of the experiments At Rutherford's behest, Geiger and Marsden performed a series of experiments where they pointed a beam of alpha particles at a thin foil of metal and, using the scintillation method devised by Crookes, Elster, & Geitel measured the scattering pattern by the use of a fluorescent screen. They spotted one out of every several thousand alpha particles bouncing off the metal foil in all directions, some right back at the source. This should have been impossible according to Thomson's model; the alpha particles should have all gone straight through. Obviously, those particles had encountered an electrostatic force far greater than Thomson's model suggested they would. Furthermore, only a small fraction of the alpha particles were deflected by more than 90°. Most flew straight through the foil with negligible deflection. To explain this bizarre result, Rutherford imagined that the positive charge of the atom was concentrated in a tiny nucleus at its center, which in turn meant that most of the atom's