(Q-2)   The Atomic Structure of Matter

    Around 1900, the physics of macroscopic objects (sticks and stones, bricks, bones... etc.) was thought to be well understood (except, as Lord Kelvin supposedly remarked, for a few details..). Motions obeyed Newton's Laws, which explained the motion of celestial objects with remarkable accuracy. Electricity obeyed the laws of Ampere, Faraday and Maxwell, and light was revealed to be an electromagnetic wave, an identification underscored by the discovery of radio waves by Heinrich Hertz, who generated them from a rapidly alternating electrical current. Technology closely followed this new physical understanding and made it pay handsomely, giving around the year 1900 electric motors, steam turbines, telegraphs and telephones, ice-making machinery, airships and more.

    Underlying it all were atoms. All material objects seemed made up of particles too small to be seen, but whose existence could be deduced from a host of subtle phenomena. Phenomena associated with the atoms themselves, however, were not at all understood, e.g. the physics behind chemical bonding, by which atoms combined to form molecules.

    Where physicists confront the unknown and need to test various explanations, they look for evidence which can be observed with great precision, like the motions of planets, which provided a sensitive test of Newton's theories. In studying atoms, precise information seemed to be contained in the wavelengths of spectral lines, of sharply defined colors emitted by atoms in a glowing gas, each of which associated with a specific kind of atom. Thus sodium glowed in twin "lines" in orange-yellow, closely spaced (they are called "lines" because that is how they appear when observed in a spectrograph, which separates light coming from a narrow slit into wavelengths). Hydrogen glowed mainly in red, and sunlight contained a yellow line (close to the sodium twins, actually) ultimately ascribed to a new element "helium" (Helios is Sun in Greek). Helium was isolated on Earth in 1895.

    Spectrometers could be teamed up with sophisticated optical "interferometers" based on the wave nature of light (touched upon in an optional section at the end of the lesson plan for section S-4). These determined the wavelengths of those emissions with amazing precision--so accurately, that at one time the standard of length, the "meter," was redefined in terms of the wavelength of a certain spectral feature. Previously it had been defined as the distance between two scratches on a metal bar kept in a vault in Paris, but the new definition could be reproduced with greater accuracy, locally at any well-equipped lab. Many thousands of wavelengths were measured, emitted by the various atomic elements, and they seemed to tell something about the atoms which emitted them. However, the "classical physics" of the 1800s had no clue about interpreting their values.

    When the existence of atoms was first recognized, some viewed them as hard little spheres bouncing off each other in a gas, or else, vibrating in unison when joined together in a regular array in a crystal. Viewing the atoms of a gas as colliding spheres, in particular, explained very well the theory of heat and the classical gas laws--even the uneven distribution of molecular (or atomic) velocities in a hot gas (calculated by Maxwell, his "Maxwellian distribution") and sophisticated ideas like the second law of thermodynamics.

Electricity and Atoms

    Yet atoms had many more properties, suggesting they contained electrically charged parts. Chemical compounds dissolved in water could often be separated when an electric current passed through them, e.g. water itself could be separated into hydrogen and oxygen. Such a process became known as electrolysis, and its laws were traced by Michael Faraday. The Swedish scientist Svante Arrhenius correctly guessed that in some chemicals, when dissolved in water, at least some of the molecules fell apart into electrically charged "ions."

    When electric forces are introduced into such a watery solution, they pull positive ions in one direction, negative ones in the opposite one. This not only carries an electric current, but also, different parts of the compound are attracted to the two electrical contacts where the current enters or leaves the solution, allowing different parts of a chemical compound to be separated chemically (unless they immediately react chemically with the water, as happens when one tries to separate a solution of table salt). In later years, in electric discharges in rarefied gases, physicists managed to separate tiny negatively charged electrons, as well as positive ions, the atomic or molecular fragments left behind when electrons were detached.

    All these suggested that a deeper level of physics existed, governing behavior on an atomic scale. Initially it was expected that the laws of Newton and Maxwell would also hold there: different players, perhaps--atoms, molecules, ions and electrons--but the same rules. It took about 30 years (1900-1930) before a new generation of physicists realized how the rules also changed as one approached atomic dimensions, and before new rules were found that replaced them.

    Repeating an earlier point: this overview is non-mathematical. It may help reveal the overall pattern, but to put it to any use, mathematics must be applied, at a fairly high level. If you never need to apply quantum physics, you will probably find here all you want to know. If you plan to study physics in greater depth, you will need more, but even then, this introduction will tell you how some of the pieces relate to each other and provide a good starting point for what comes next.

The Balmer Series

        (section adapted from an optional historical note in Lsun5wav.htm)

The first clue to the meaning of spectral lines came in 1885 from Johann Balmer, a high school math teacher in Basel, Switzerland. Of all the atomic spectra, the simplest is that of hydrogen--not surprising, since hydrogen is the smallest of atoms, and presumably the simplest. In an electric discharge (similar to that of a neon lamp) it emits just one series of spectral lines, of which four usually register on the photographic record of a spectrograph (they were labeled by the first 4 Greek letters, as in the image above-- α, β, γ and δ, in order of descending wavelength). As noted, the wavelength λ (lambda) of each emitted color had been quite accurately measured, and Balmer discovered that the values all fit a simple formula:

where n = 3,4,5.. and R is the experimentally obtained "Rydberg constant," named for Johann Rydberg, a Swedish physicist who evaluated it. (A translation of Balmer's original article exists on the web.)

    The lowest of these "lines" (n=3) is the red "hydrogen alpha" line (Hα for short), contributing the dominant red color of the solar chromosphere. Most sunlight originates in the photosphere, the outermost layer of the sun as seen by the eye; light in deeper layers is just reabsorbed near the place where it was emitted. The next layer out, the chromosphere, glows very dimly in red, a glow originally seen only during a total eclipse of the Sun, when the Moon blocked the much brighter light of the photosphere.

    The chromosphere emits relatively little light, and its contribution is normally drowned out by the much greater brightness of the photosphere. It becomes visible during a total eclipse of the Sun. Then, after the Moon completely covers the photosphere, a reddish glow becomes visible around the Sun, in a relatively narrow ring; above it is the corona whose light is even fainter.

    The chromosphere is important because it is the site of sudden energy releases associated with sunspot magnetism--the so called solar flares. Flares only rarely show themselves in white light, as brightenings against the background of the photosphere (one such rare event was the first flare to be observed, seen by Richard Carrington in 1859). But they are easily seen when viewed through a sensitive filter which only transmits the narrow Hα line and blocks everything else. Through such filters, flare activity and many other solar phenomena are regularly monitored and photographed.

    After Balmer announced his series, Lyman found in the ultra-violet a series of lines

of which the "Lyman α" line is particularly prominent in the glow of the Earth's outer atmosphere, photographed by astronauts from the Moon. Also, Paschen found a series of lines in the infra-red

suggesting they all belonged to a single family with

The regularities of these series seemed like a clue to processes inside the atom, responsible for the emission of narrowly defined colors, or "spectral lines." But what was the message?

Or else, return to section #6 on physics related to the Sun: (S-6) Seeing the Sun in a New Light

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Author and Curator:   Dr. David P. Stern
     Mail to Dr.Stern:   stargaze("at" symbol)phy6.org