The correct answer is Ernest Rutherford. Explanation & Key Takeaways: • In 1911, the British physicist Ernest Rutherford conducted the famous alpha-particle scattering experiment, also known as the Gold Foil Experiment, which fundamentally changed the understanding of atomic structure. • The experiment involved bombarding a very thin gold foil (approximately 1000 atoms thick) with fast-moving alpha particles, which are doubly-charged helium ions possessing a mass of 4u and high energy. • Rutherford observed that most alpha particles passed straight through the foil, some were deflected by small angles, and a very small fraction (about 1 in 12,000) rebounded almost 180 degrees. • From these observations, he concluded that the atom contains a very small, dense, and positively charged center called the nucleus, where nearly all the mass of the atom is concentrated. • He proposed the Nuclear Model of the Atom, stating that electrons revolve around the nucleus in well-defined circular paths, much like planets around the sun. Related Concepts: • J.J. Thomson: • In 1897, he discovered the electron using cathode ray tube experiments. • In 1904, he proposed the Plum Pudding Model, which suggested that an atom is a sphere of positive charge with negatively charged electrons embedded in it like raisins in a pudding. • John Dalton: • In 1808, he presented Dalton’s Atomic Theory, which stated that all matter is made of indivisible particles called atoms. • He theorized that atoms of the same element are identical in mass and chemical properties, forming the basis of modern chemistry. • Niels Bohr: • In 1913, he modified Rutherford's model to overcome the problem of atomic stability. • He proposed the Bohr Model, stating that electrons revolve in discrete orbits (fixed energy levels) called shells (K, L, M, N), and they do not radiate energy while in these orbits. • Gold Foil Selection: • Gold was chosen for the experiment because of its high malleability, allowing it to be hammered into an extremely thin sheet, which was necessary to observe the scattering of individual alpha particles. Important Notes: • Rutherford is widely regarded as the "Father of Nuclear Physics" for his discovery of the atomic nucleus. • A major drawback of the Rutherford model was that it could not explain the stability of the atom; according to classical electrodynamics, an accelerating electron should continuously lose energy and eventually spiral into the nucleus. • The discovery of the nucleus led to the later discovery of the proton, a term coined by Rutherford in 1920 to describe the hydrogen nucleus as a fundamental particle.