Hypothesis according to Aristotle (350 BC): All matter consists of the elements earth, air, fire, and water.
Experiment: A tree grows from the earth as water is added. The tree is then set on fire in an effort to form new elements.
1. What kind of results would be observed from this experiment? Fire, ash (earth), smoke (air)
2. Would these observations support or reject the hypothesis? Explain. Support. No elements have been produced that are something other than the four elements from the hypothesis.
New information: Hooking up two wires to opposite electrodes of a battery and submerging the free ends of the wires into water causes bubbles to form on each wire. The wire hooked up to the negative electrode always produces exactly twice the volume but only 1/8 of the mass as the gas from the positive electrode.
3. How does this new information support the original hypothesis? Air is formed from the water.
4. How does this new information contradict the original hypothesis? Clearly, we have two different types of air. The smaller volume of air is much heavier than the larger volume of air. Air cannot be a single element.
New hypothesis according to Dalton (1803): All matter is composed of tiny, indivisible, electrically neutral spheres called atoms. Atoms of the same element are all identical. Atoms of different elements have different masses.
Experiment: Neutral atoms sealed in a cathode ray tube produce a beam of particles that are attracted to the positive end of a magnet, but repelled by the negative end.
5. How does this experiment contradict Dalton's hypothesis? Dalton said that atoms are indivisible, meaning they are the smallest particle in existence. The negatively charged beam of particles could only have come from the neutral atoms meaning that there must be particles smaller than the atom itself.
Experiment: A sample of neon gas is passed through a mass spectrometer. The spectrometer records that 82% of the atoms in the sample have a mass of 20 u. The other 18% have a mass of 18 u.
6. What do we call atoms of the same element that have different masses? Isotopes.
7. How does this experiment contradict Dalton's hypothesis? Atoms of the same element are not always identical.
8. Propose a new hypothesis for the structure of an atom. Hint: How can an atom be neutrally charged and contain charged particles at the same time? The atom must be made up of both positively charged and negatively charged particles. Your hypothesis should describe how those particles might be arranged.
9. Compare your model to that of J.J. Thomson (p.110) proposed in 1897. Thomson proposed that the electrons were like chocolate chips embedded in a positively charged ball of cookie dough.
New information: Tiny positively charged alpha particles shot at close to the speed of light at a thin sheet of gold foil passed straight through without being deflected about 99.99% of the time. About 1 in every 20,000 alpha particles was repelled straight back. The only way this could happen is if the alpha particle encountered a very large positive charge in an extremely small region of the atom.
10. Revise your hypothesis from #8 as needed. All of the positively charged particles must be crammed into a really tiny nucleus.
Hypothesis according to Rutherford (1911): An atom consists of a very dense, positively charged nucleus with the electrons roaming through the rest of the atom, which is mostly empty space.
New information: Neils Bohr viewed the bright-line spectra produced by gaseous atoms that were energized. He reasoned that the lines represented specific "jumps" by the excited electrons.
11. How does this information contradict Rutherford's hypothesis? Electrons cannot roam at random. They must have a definite structure (rings).
Hypothesis according to Bohr (1913): The nucleus contains protons and neutrons while the electrons orbit the nucleus in distinct rings. The rings are like rungs on a ladder. The electrons can jump from one ring to another, but can never stay in between adjacent rings.
New information: X-ray diffraction allows us to determine the 3-D shaped of molecules. We know the bonds form when electrons are shared among different atoms. The ring structure described by Bohr cannot account for the shapes of molecules.
New hypothesis (current): Electrons reside in orbitals of a specific shape (electron clouds). These orbitals and their orientations determine the 3-D shape of a molecule.
12. Why do you suppose Aristotle's hypothesis lasted for thousands of years while the more accurate hypotheses have lasted for much shorter periods? We did not have the technology to examine extremely small particles back in 400 B.C.
As technology continues to advance, our ability to peer inside the world of atoms also becomes more advanced.
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