The Structure of the Atom Chapter 3.2 The Structure of the Atom

Discovery of the Electron The first discovery of a subatomic particle resulted from investigations into the relationship between electricity and matter. In the late 1800s, many experiments were performed in which electric current was passed through various gases at low pressures. These experiment were carried into glass tube knowns as cathode-ray tubes.

Cathode ray and Electrons Observation: investigators notice that when current was passed through a cathode-ray tube, the surface of the tube directly opposite the cathode glowed. Hypothesis: they hypothesized that the glow was caused by a stream of particle which they called a cathode ray. The ray travel from the cathode to the anode when current pass through the tube. Experiment : carried out by Joseph John Thompson (English physicist) in 1897 Joseph John Thompson was curious about the composition of the cathode ray :wanted to know what the cathode ray were, where they came from , and if they had any mass or charge.

Cathode ray tube

Cathode Ray tube Experiment

Plum Pudding Model Conclusion: Because atoms are electrically neutral, they must contain a positive charge to balance the negative electrons Because electrons have so much less mass than atoms, atoms must contain other particles that account for most of their masses. Thompson then proposed the Plum Pudding model.

Discovery of the Atomic Nucleus Thompson model was disproved in 1911 by Earnest Rutherford and his associates Hans Geiger and Earnest Marsden. The scientist bombarded a piece of gold with alpha particles (positively charged particles with about 4 times the mass of a hydrogen atom) They expected alpha particles to pass through with only a slight deflection.

Discovery of the Atomic Nucleus Rutherford concluded that the force must be caused by a very densely packed bundle of matter with positive electric charge. Rutherford called this positive bundle of matter the nucleus. He also discovered that the volume of a nucleus was very small compared with the total volume of an atom. (if the nucleus was the size of a marble than an atom would be the size of a football field)

Composition of the Atomic Nucleus Scientist then concluded that all atomic nuclei are made up of two kinds of particles :Protons and neutrons. (with the exceptions of hydrogen atom) Proton (+) Electron (-) Neutrons (neutral)

The atom Forces in the nucleus: Generally, particles that have the same charge repel one another. When two protons are extremely close to each other , there is a strong attraction between them. (same attraction for neutrons) The forces that holds the nuclear particles together (p-p, n-n, & p-n) are referred to as nuclear forces. Because the radii are so small they are expressed in picometer (1pm= 1x10^-12 m ) 0.000000000001m

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