Modelos de THOMSON y RUTHERFORD. ¿CÓMO descubrieron los protones y electrones?

Introduction to Experiments and Research

Importance of Reasoning in Research

• The speaker emphasizes that the best researchers are not those who memorize facts but those who can reason and design experiments.

• Highlights the significance of understanding how great thinkers, like Thomson, approached their work.

Thomson's Contributions to Atomic Theory

William Crookes' Experiment

• Introduces William Crookes, an English physicist who designed a vacuum tube experiment demonstrating cathode rays.

• Describes how Crookes discovered luminescence produced by electric current flowing from the cathode (negative plate) to the anode (positive plate).

Thomson's Enhancements

• Thomson improved upon Crookes' design with better vacuum technology, allowing for more precise analysis of cathode rays.

• He modified the experiment by adding a hole in the anode and a measurement screen to track cathode ray paths.

Discoveries About Cathode Rays

Properties of Cathode Rays

• Thomson demonstrated that cathode rays possess mass, differentiating them from light rays which do not exert force.

• Conducted experiments showing that cathode rays are negatively charged as they were attracted to positively charged plates.

Mathematical Relationships

• Explains how increasing electric field strength resulted in greater deflection of the rays, establishing a proportional relationship between charge and deviation.

Significance of Mass-to-Charge Ratio

Limitations in Measurement

• Discusses how measuring only the mass-to-charge ratio limited understanding; individual particle properties remained unknown.

Millikan's Oil Drop Experiment

• Robert Millikan later determined both charge and mass of electrons through his oil drop experiment, revealing they had equal charge magnitude as hydrogen ions but much less mass.

Discovery of Electrons

Thomson’s Model of Atom

• After discovering electrons, Thomson theorized about positive charges within atoms to balance out negative charges.

Understanding Rutherford's Atomic Model

Disagreement with Thomson's Model

• The physicist Ernest Rutherford disagreed with Thomson's atomic model, citing unresolved doubts and differences in expertise regarding electromagnetism and radiation.

Experiment Setup

• To test the atomic model, Rutherford used alpha particles emitted from a radiation source, requiring protective measures due to their high energy and positive charge.

• A thin gold foil was chosen for the experiment because gold atoms are large; using a very thin layer ensured that only one or two atoms would be hit during the experiment.

Observations from the Experiment

• If Thomson's model were correct, alpha particles should pass through the gold foil with minimal deflection. However, Rutherford expected some deviation based on his hypothesis of a dense nucleus.

• Contrary to expectations, while most alpha particles passed through without significant deviation, some were deflected at large angles or even bounced back towards their source.

Conclusions Drawn by Rutherford

• Rutherford was astonished by these results, likening it to firing a 15-inch shell at tissue paper and having it bounce back. This led him to propose that there must be something much denser within the atom.

• He theorized that an atom contains a small but dense positively charged nucleus where protons reside, surrounded by electrons orbiting around it.

Implications of Findings

• The majority of an atom is empty space; thus, most alpha particles could pass through without interaction unless they collided directly with the nucleus.

• Later research confirmed Rutherford’s findings about atomic structure and allowed for measurements of atomic sizes relative to their nuclei.

Final Thoughts on Protons and Atomic Structure

• Rutherford proposed that protons account for the positive charge in the nucleus. It was later established that protons have similar mass and charge as hydrogen ions (protons).

• In conclusion, he suggested a new atomic model where electrons orbit around a small but positively charged nucleus made up of protons.

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