Chapter 25, Problem 25.35P

In 1911, Ernest Rutherford and his assistants Geiger and Marsden conducted an experiment in which they scattered alpha particles (nuclei of helium atoms) from thin sheets of gold. An alpha particle, having charge +2e and mass 6.64 × 10^-27 kg, is a product of certain radioactive decay's. The results of the experiment led Rutherford to the idea that most of an atom’s mass is in a very small nucleus, with electrons in orbit around it. (This is the planetary model of the atom, which we’ll study in Chapter 42.) Assume an alpha particle, initially very far from a stationary gold nucleus, is fired with a velocity of 2.00 × 10⁷ m/s directly toward the nucleus (charge +79e). What is the smallest distance between the alpha particle and the nucleus before the alpha particle reverses direction? Assume the gold nucleus remains stationary.