7 Explain how Rutherford's students knew that Thomson's model of the atom needed to change.

Rutherford's students recognized that Thomson's "plum pudding" model of the atom was inadequate based on the experimental evidence gathered from Rutherford's gold foil experiments. Here's a detailed explanation of how they came to this conclusion: ### **Thomson's Plum Pudding Model** - **Structure**: Proposed by J.J. Thomson, this model depicted the atom as a diffuse cloud of positive charge with negatively charged electrons embedded randomly within it, much like plums within a pudding. - **Assumptions**: The positive charge was spread out uniformly throughout the atom, and electrons were relatively light and mobile within this positive matrix. ### **Rutherford's Gold Foil Experiment** - **Setup**: Conducted by Hans Geiger and Ernest Marsden under Rutherford's supervision, the experiment involved directing a beam of alpha particles (positively charged) at a thin gold foil. - **Observations**: - **Most** alpha particles passed straight through the foil with little or no deflection, supporting the idea that atoms are mostly empty space. - **A small fraction** of the alpha particles were deflected at large angles, and a very few even bounced back toward the source. ### **Implications of the Observations** - **Contradiction with Thomson's Model**: - **Uniform Positive Charge**: If the positive charge were spread out as in the plum pudding model, large deflections of alpha particles would be exceedingly rare. The occasional slight deflections fit the model, but the significant deflections could not be explained. - **Dense Nucleus Presence**: The unexpected large-angle deflections suggested the existence of a highly concentrated positive region within the atom—a nucleus—where most of the atom's mass and positive charge are located. ### **Conclusion by Rutherford's Students** - **Need for a New Model**: The experimental results indicated that the positive charge in an atom is not diffuse but instead concentrated in a small, dense nucleus. This finding directly contradicted the plum pudding model. - **Development of the Nuclear Model**: Recognizing these discrepancies, Rutherford and his students proposed a new atomic model. In this model, electrons orbit a central, dense nucleus, which contains most of the atom's mass and positive charge. This model laid the groundwork for the modern understanding of atomic structure. ### **Significance** - **Shift in Atomic Theory**: The recognition that atoms have a central nucleus fundamentally changed the scientific community's understanding of atomic structure, leading to the development of the planetary model of the atom and later advancements in quantum mechanics. In summary, the unexpected deflections of alpha particles in Rutherford’s experiments provided clear evidence that the positive charge in atoms is concentrated in a small nucleus rather than being spread out as Thomson’s model proposed. This compelling evidence drove Rutherford's students to conclude that a new atomic model was necessary to accurately describe the structure of atoms.