(LS-7A)   The Discovery of Atoms and Nuclei

What follows is a brief list of benchmarks and discoveries that shaped our ideas on atoms and nuclei. It supplements lesson plan Lsun7erg, attached to section S-7 "The Energy of the Sun""From Stargazers to Starships."

(The table below may be written by the teacher on the board and copied by the students, with the comment "this will not be on any test." Or else, paper copies may be distributed. Compare this list to a longer one on another web site.)

Listed below are a few comments on each item on the list. To do justice to this subject might require a one-semester university course.

• The notion of atoms started from a fundamental problem in chemistry (#1): why did (say) one gram of hydrogen always combine with 8 grams of oxygen, never more, never less? Because each molecule of the resulting compound--water--always contained a fixed number of atoms of each kind. By comparing different reactions, Dalton concluded that (for instance) 2 atoms of hydrogen combined with one of oxygen, to create H₂O.

• Avogadro (#2, #5), in Italy, meanwhile noted a simple relation between the volume and weight of gases. Quantities which from chemistry seemed to combine naturally--1 gr. of hydrogen, 16 gr. of oxygen, 35.5 gr. of chlorine etc.--had the same volume, and Avogadro proposed that they contained the same number of molecules or atoms.

  (He also proposed that atoms in gases often combined in pairs, to create molecules such as H₂, O₂ and Cl₂, explaining some strange factors of two. We thus should talk about the number of molecules in 2 gr. of hydrogen, 32 gram of oxygen, 71 gram of chlorine, etc. That number is now known as Avogadro's number, and is truly enormous.)

  Avogadro's work was neglected for many years, while chemists struggled to understand the way atoms combined. At a conference of chemists in 1860, Cannizzaro (#5) again drew attention to Avogadro's results, and after that progress was rapid.

• Meanwhile, clues accumulated suggesting that atoms carried electric charges. Humphrey Davy (#3) used an electric current to separate new elements out of molten salts (a process called electrolysis). He obtained sodium and then potassium, soft metals which burned violently.

• Faraday, who started as Davy's assistant, derived in 1833 the laws of electrolysis (#4), which suggested that in a water solution (and also a molten salt), each atom or molecular fragment carried a fixed electric charge.

• Other researchers studied the flow of electricity in rarefied gases, under the influence of high voltage (fluorescent lamps are one outgrowth of such experiments). It became evident that such currents were carried by positive and negative particles in the gas. Joseph ("J.J.") Thompson isolated one type, a very light negative particle, measured its properties and named it the electron. (#7).

• The conducting gases also contained positive "ions" (wanderers) which J.J. Thompson studied as well. Such ions ("alpha particles," actually nuclei of helium) were also emitted by heavy radioactive elements, discovered in 1895 (#6).

• Starting in 1910, Robert Millikan at the Calif. Inst. of Technology accurately measured the charge of the electron (#9), by spraying tiny droplets of oil from an atomizer into a region of electric forces, between two parallel horizontal plates. Some of the droplets became electrically charged by friction or radioactivity, usually by no more than 1-2 electrons, and fell more slowly in the field of an observing (horizontal) microscope. By adjusting the voltage, Millikan could stop them, so that electric force exactly balanced weight. That force could be derived from the voltage, while the weight was obtained by turning off the voltage and timing the free fall, governed mainly by air resistance.
          The one unknown left was the electric charge, which could now be computed. Comparing this to Faraday's results, one could get "Avogadro's number", the number of (say) hydrogen atoms in one gram of hydrogen--an enormously large number. The size of atoms was finally clearly determined! For Millikan's 1913 report of his discovery, see here.

• Ernest Rutherford, born on New Zealand, showed in 1911 that alpha particles were sometimes very strongly scattered by the positive charges of the atom, in a way that could only be explained if such charges were concentrated in a very small volume, practically a point in space. He therefore suggested that every atom had a compact nucleus, with negative electrons floating around it (#8).

  Rutherford's picture suggested that the nucleus was like a miniature Sun, with electrons orbiting it like planets. If Newton's laws were valid on the atomic scale, that might indeed be so, but as later research showed, on the atomic distance scale Newton's laws change into other forms. By these new laws of "quantum mechanics," electrons do not move in precisely defined orbits, but are distributed in space in a way that only allows the probability of finding them anywhere to be calculated. Similarly, energized atoms are only allowed to exist in one of a number of energy levels.

• There remained a problem: nuclei were too heavy. Helium nuclei had twice the charge of the proton but 4 times the mass. For a while scientists wondered whether helium nuclei contained 4 protons and 2 electrons. Then in 1932 Chadwick discovered the neutron and it was realized that helium nuclei contained two protons, two neutrons and no electrons. One form of nuclear force ("the weak nuclear force") controlled the ratio of neutrons to protons.

• In 1938 Hans Bethe proposed that the Sun obtained its energy by fusing hydrogen nuclei to form helium. He also showed how fusion could proceed by means of a cyclical process, involving nuclei of carbon and nitrogen; the carbon and nitrogen nuclei are recovered and the only change is that 4 hydrogen atoms combine into helium. Today Bethe's cycle is believed to operate mainly in stars somewhat hotter than the Sun.

  Back to lesson plan Lsun7erg.