This lesson explores how scientists discovered that atoms contain smaller particles and how early models attempted to explain atomic structure. Thomson’s and Millikan’s experiments established key properties of the electron and showed that atoms are divisible.
In the late 1800s, scientists investigated electrical discharges in low-pressure gases. J. J. Thomson used a cathode ray tube, a sealed glass tube containing two metal electrodes, with most of the air removed. When a high voltage was applied, a visible beam called a cathode ray appeared.
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What is a cathode ray tube?
It is a sealed glass tube with most air removed and two metal electrodes. Applying a high voltage produces a visible cathode ray between the electrodes.
What defines isotopes of the same element?
They have the same number of protons but different numbers of neutrons, giving them different masses.
What did Millikan’s oil-drop experiment determine?
It showed that electric charges on oil droplets were whole-number multiples of a fundamental charge, whose magnitude is approximately 1.602×10−19C—the magnitude of an electron’s charge.
How was the electron’s mass determined?
Scientists combined Millikan’s measurement of the electron’s charge with Thomson’s charge-to-mass ratio. The resulting mass is approximately 9.107×10−31kg.
How did the plum pudding model represent an atom?
Thomson pictured the atom as a spread-out positive charge with negative electrons embedded in it. The positive and negative charges balanced, making the atom electrically neutral.
Quiz questions
What observation in Thomson’s cathode ray experiments indicated that the rays carried negative charge?Answer: They bent toward a positively charged plate and away from a negatively charged plate.
Why did Thomson infer that cathode ray particles were present in all atoms?Answer: The rays behaved the same when the electrodes were made of different metals.
What pattern did Millikan observe in the charges of oil droplets?Answer: Each droplet’s charge was a whole-number multiple of a smallest charge.
What did the rare, large-angle deflections in Rutherford’s gold foil experiment suggest?Answer: Positive charge was concentrated in a small, dense region of the atom.
Which feature distinguishes isotopes of one element from atoms of another element?Answer: Isotopes have the same number of protons but different numbers of neutrons.
Based on Chemistry 2e by Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson, PhD (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.