2. Atomic Structure

The script begins with an introduction to MIT OpenCourseWare, highlighting its mission to provide free access to high-quality educational resources. The speaker, Catherine Drennan, encourages donations to support this initiative and directs interested parties to the MIT OpenCourseWare website. She then sets the stage for an interactive classroom environment, using incentives like MIT chemistry T-shirts to encourage student participation. The lecture's focus is announced as the discovery of the electron and the nucleus, with a promise to blend historical context with modern applications in chemistry.

This paragraph delves into the historical aspect of chemistry, discussing the discovery of subatomic particles like the electron and the nucleus. Drennan describes the scientific community's mindset in the late 1890s, which was marked by a sense of completion in understanding the universe through atomic theory and Newtonian mechanics. However, she emphasizes the imminent major discoveries that challenged these notions, starting with J.J. Thompson's experiment with cathode rays, which led to the identification of negatively charged particles, later known as electrons.

The summary of J.J. Thompson's cathode ray experiment is presented, detailing how he used an evacuated glass cylinder filled with hydrogen gas and observed rays upon applying a current. Thompson's curiosity about the nature of these rays led him to conduct experiments that ultimately revealed their deflection by electric fields, indicating the presence of negatively charged particles. The deflection patterns allowed Thompson to deduce the ratio of charge to mass for these particles, which were later identified as electrons, with a significantly smaller mass compared to the positively charged particles, suggesting the existence of atoms smaller than previously thought.

The narrative shifts to Ernest Rutherford's experiments with radioactive materials, including his collaboration with Hans Geiger and E. Marsden. Their gold foil experiment, involving alpha particles, led to the unexpected discovery of backscattering, challenging the then-current atomic model. The observation that a small fraction of alpha particles bounced back upon hitting the gold foil indicated the presence of a dense, positively charged center within the atom, which Rutherford named the nucleus. This finding contradicted the 'plum pudding' model and laid the groundwork for the Rutherford model of the atom.

The script outlines a classroom experiment designed to replicate Rutherford's findings, using a piece of gold foil and alpha particles represented by ping pong balls. The objective is for students to observe backscattering events and use these observations to calculate the diameter of the nucleus. The experiment is a hands-on approach to understanding the atomic structure and serves as a practical application of theoretical knowledge.

A detailed explanation of how to calculate the diameter of the nucleus using the backscattering data from the classroom experiment is provided. The process involves understanding the probability of backscattering and relating it to the area occupied by the nuclei versus the entire atom's area. The calculation uses the measured area of the foil, the counted number of nuclei, and the observed backscattering events to estimate the nucleus's size.

The script concludes with a discussion on the stability of atoms, questioning why electrons do not crash into the nucleus despite the attractive force predicted by Coulomb's Law. The lecturer uses Newton's Second Law to illustrate the expected rapid collapse of an atom if classical mechanics were solely applicable. The paragraph highlights the limitations of classical physics in explaining atomic behavior and introduces the need for quantum mechanics to accurately describe the subatomic world.