(A short history of the knowledge of the atom) Compiled by Jim Walker
Originated: Sept. 1988 Latest revision: Nov. 2004
atom n. A unit of matter, the smallest unit of an element, consisting of a dense, central, positively charged nucleus surrounded by a system of electrons, equal in number to the number of nuclear protons, the entire structure having an approximate diameter of 10-8 centimeter and characteristically remaining undivided in chemical reactions except for limited removal, transfer, or exchange of certain electrons.
The history of the study of the atomic nature of matter illustrates the thinking process that goes on in the philosophers and scientists heads. The models they use do not provide an absolute understanding of the atom but only a way of abstracting so that they can make useful predictions about them. The epistemological methods that scientists use provide us with the best known way of arriving at useful science and factual knowledge. No other method has yet proven as successful.
In the beginning
Actually, the thought about electricity came before atoms. In about 600 B.C. Thales of Miletus discovered that a piece of amber, after rubbing it with fur, attracts bits of hair and feathers and other light objects. He suggested that this mysterious force came from the amber. Thales, however, did not connect this force with any atomic particle.
Not until around 460 B.C., did a Greek philosopher, Democritus, develop the idea of atoms. He asked this question: If you break a piece of matter in half, and then break it in half again, how many breaks will you have to make before you can break it no further? Democritus thought that it ended at some point, a smallest possible bit of matter. He called these basic matter particles, atoms.
Unfortunately, the atomic ideas of Democritus had no lasting effects on other Greek philosophers, including Aristotle. In fact, Aristotle dismissed the atomic idea as worthless. People considered Aristotle's opinions very important and if Aristotle thought the atomic idea had no merit, then most other people thought the same also. (Primates have great mimicking ability.)
For more than 2000 years nobody did anything to continue the explorations that the Greeks had started into the nature of matter. Not until the early 1800's did people begin again to question the structure of matter.
In the 1800's an English chemist, John Dalton performed experiments with various chemicals that showed that matter, indeed, seem to consist of elementary lumpy particles (atoms). Although he did not know about their structure, he knew that the evidence pointed to something fundamental.
In 1897, the English physicist J.J. Thomson discovered the electron and proposed a model for the structure of the atom. Thomson knew that electrons had a negative charge and thought that matter must have a positive charge. His model looked like raisins stuck on the surface of a lump of pudding.
In 1900 Max Planck, a professor of theoretical physics in Berlin showed that when you vibrate atoms strong enough, such as when you heat an object until it glows, you can measure the energy only in discrete units. He called these energy packets, quanta.
Physicists at the time thought that light consisted of waves but, according to Albert Einstein, the quanta behaved like discrete particles. Physicists call Einstein's discrete light particle, a "photon*."
Atoms not only emit photons, but they can also absorb them. In 1905, Albert Einstein wrote a ground-breaking paper that explained that light absorption can release electrons from atoms, a phenomenon called the "photoelectric effect." Einstein received his only Nobel Prize for physics in 1921 for his work on the photoelectric effect.
* Note: I anachronistically use the word photon here. Actually, physicists did not refer to light quanta as photons until after Gilbert N. Lewis proposed the name in an article in Nature, Vol 118, Pt. 2, December 18, 1926.
A heated controversy occured for many years on deciding whether light consisted of waves or particles. The evidence appeared strong for both cases. Later, physicists showed that light appears as either wave-like or particle-like (but never both at the same time) depending on the experimental setup.
Other particles got discovered around this time called alpha rays. These particles had a positive charge and physicists thought that they consisted of the positive parts of the Thompson atom (now known as the nucleus of atoms).
In 1911 Ernest Rutherford thought it would prove interesting to bombard atoms with these alpha rays, figuring that this experiment could investigate the inside of the atom (sort of like a probe). He used Radium as the source of the alpha particles and shinned them onto the atoms in gold foil. Behind the foil sat a fluorescent screen for which he could observe the alpha particles impact.
The results of the experiments came unexpected. Most of the alpha particles went smoothly through the foil. Only an occasional alpha veered sharply from its original path, sometimes bouncing straight back from the foil! Rutherford reasoned that they must get scattered by tiny bits of positively charged matter. Most of the space around these positive centers had nothing in them. He thought that the electrons must exist somewhere within this empty space. Rutherford thought that the negative electrons orbited a positive center in a manner like the solar system where the planets orbit the sun.
Rutherford knew that atoms consist of a compact positively charged nucleus, around which circulate negative electrons at a relatively large distance. The nucleus occupies less than one thousand million millionth (10 ) of the atomic volume, but contains almost all of the atom's mass. If an atom had the size of the earth, the nucleus would have the size of a football stadium.
Not until 1919 did Rutherford finally identify the particles of the nucleus as discrete positive charges of matter. Using alpha particles as bullets, Rutherford knocked hydrogen nuclei out of atoms of six elements: boron, fluorine, sodium, aluminum, phosphorus, an nitrogen. He named them protons, from the Greek for 'first', for they consisted of the first identified building blocks of the nuclei of all elements. He found the protons mass at 1,836 times as great as the mass of the electron.
But there appeared something terribly wrong with Rutherford's model of the atom. The theory of electricity and magnetism predicted that opposite charges attract each other and the electrons should gradually lose energy and spiral inward. Moreover, physicists reasoned that the atoms should give off a rainbow of colors as they do so. But no experiment could verify this rainbow.
In 1912 a Danish physicist, Niels Bohr came up with a theory that said the electrons do not spiral into the nucleus and came up with some rules for what does happen. (This began a new approach to science because for the first time rules had to fit the observation regardless of how they conflicted with the theories of the time.)
Bohr said, "Here's some rules that seem impossible, but they describe the way atoms operate, so let's pretend they're correct and use them." Bohr came up with two rules which agreed with experiment:
RULE 1: Electrons can orbit only at certain allowed distances from the nucleus.
RULE 2: Atoms radiate energy when an electron jumps from a higher-energy orbit to a lower-energy orbit. Also, an atom absorbs energy when an electron gets boosted from a low-energy orbit to a high-energy orbit.
The electron can exist in only one of the orbits. (The diagram shows only five orbits, but any number of orbits can theoretically exist.)
Light (photons) emit whenever an electron jumps from one orbit to another. The jumps seem to happen instantaneously without moving through a trajectory.
The examples above show only two possibilities from Rule 2.
By the 1920s, further experiments showed that Bohr's model of the atom had some troubles. Bohr's atom seemed too simple to describe the heavier elements. In fact it only worked roughly in these cases. The spectral lines did not appear correct when a strong magnetic field influenced the atoms.
Bohr- Sommerfeld model of the atom
Bohr and a German physicist, Arnold Sommerfeld expanded the original Bohr model to explain these variations. According to the Bohr-Sommerfeld model, not only do electrons travel in certain orbits but the orbits have different shapes and the orbits could tilt in the presence of a magnetic field. Orbits can appear circular or elliptical, and they can even swing back and forth through the nucleus in a straight line.
The orbit shapes and various angles to the magnetic field could only have certain shapes, similar to an electron in a certain orbit. As an example, the fourth orbit in a hydrogen atom can have only three possible shapes and seven possible traits. These added states allowed more possibilities for different spectral lines to appear. This brought the model of the atom into closer agreement with experimental data.
The conditions of the state of the orbit got assigned quantum numbers. The three states discussed so far consist of: orbit number (n), orbit shape (l) and orbit tilt (m).
In 1924 an Austrian physicist, Wolfgang Pauli predicted that an electron should spin (kind of like a top) while it orbits around the nucleus. The electron can spin in either of two direction. This spin consisted of a fourth quantum number: electron spin (s).
Pauli gave a rule governing the behavior of electrons within the atom that agreed with experiment. If an electron has a certain set of quantum numbers, then no other electron in that atom can have the same set of quantum numbers. Physicists call this "Pauli's exclusion principle." It provides an important principle to this day and has even outlived the Bohr-Sommerfeld model that Pauli designed it for.
In 1924 a Frenchman named Louis de Broglie thought about particles of matter. He thought that if light can exist as both particles and waves, why couldn't atom particles also behave like waves? In a few equations derived from Einstein's famous equation, (E=mc2) he showed what matter waves would behave like if they existed at all. (Experiments later proved him correct.)
In 1926 the Austrian physicist, Erwin Schrödinger had an interesting idea: Why not go all the way with particle waves and try to form a model of the atom on that basis? His theory worked kind of like harmonic theory for a violin string except that the vibrations traveled in circles.
The world of the atom, indeed, began to appear very strange. It proved difficult to form an accurate picture of an atom because nothing in our world really compares with it.
Schrödinger's wave mechanics did not question the makeup of the waves but he had to call it something so he gave it a symbol:
The "psi" symbol of Schrödinger's wave came from the Greek lettering system.
In 1926, a German physicist, Max Born had an idea about 'psi'. Born thought they resembled waves of chance. These ripples moved along waves of chance, made up of places where particles may occur and places where no particles occured. The waves of chance ripple around in circles when the particle appears like an electron in an atomic orbit, and they ripple back and forth when the electron orbit goes straight through the nucleus, and they ripple along in straight lines when a free particle moves through interatomic space. You can think of them as waves when traveling through space and as particles whenever they travel in circles. However, they cannot exist as both waves and particles at the same time.
Just before Schrödinger proposed his theory, a German physicist Werner Heisenberg, in 1925, had a theory of his own called matrix mechanics which also explained the behavior of atoms. The two theories seemed to have an entirely different sets of assumptions yet they both worked. Heisenberg based his theory on mathematical quantities called matrices that fit with the conception of electrons as particles whereas Schrödinger based his theory on waves. Actually, the results of both theories appeared mathematically the same.
In 1927 Heisenberg formulated an idea, which agreed with tests, that no experiment can measure the position and momentum of a quantum particle simultaneously. Scientists call this the "Heisenberg uncertainty principle." This implies that as one measures the certainty of the position of a particle, the uncertainty in the momentum gets correspondingly larger. Or, with an accurate momentum measurement, the knowledge about the particle's position gets correspondingly less.
The visual concept of the atom now appeared as an electron "cloud" which surrounds a nucleus. The cloud consists of a probability distribution map which determines the most probable location of an electron. For example, if one could take a snap-shot of the location of the electron at different times and then superimpose all of the shots into one photo, then it might look something like the view at the top.
Note: Just as no map can equal a territory, no concept of an atom can possibly equal its nature. These models of the atom simply served as a way of thinking about them, albeit they contained limitations (all models do).
Although the mathematical concept of the atom got better, the visual concept of the atom got worse. Regardless, even simplistic visual models can still prove useful. Chemists usually describe the atom as a simple solar system model similar to Bohr's model but without the different orbit shapes. The important emphasis for chemistry attemps to show the groupings of electrons in orbital shells. (The example above shows the first eleven elements.)
Chemical behavior of the elements form together to create molecules. Molecules may share electrons as the hydrogen and water molecules above illustrates. (Atoms which share electrons have the name "ions.") The outer electron shell of an atom actually does the sharing and bonding of the atoms. This in turn allows chemists to describe the interactions of chemistry. Even though the orbit model of the atom does not provide an accurate model, it works well for describing chemistry.
A helium atom with two electrons orbiting a nucleon made of two protons and two neutrons
A mystery of the nature of the nucleus remained unsolved. The nucleus contains most of the atom's mass as well as the positive charge. The protons supposedly accounted for this mass. However, a nucleus with twice the charge of another should have twice the number of protons and twice the mass. But this did not prove correct. Rutherford speculated in 1920 that there existed electrically neutral particles with the protons that make up the missing mass but no one accepted his idea at the time.
Not until 1932 did the English physicist James Chadwick finally discover the neutron. He found it to measure slightly heavier than the proton with a mass of 1840 electrons and with no charge (neutral). The proton-neutron together, received the name, "nucleon."
Isotopes of Hydrogen
Although scientists knew that atoms of a particular element have the same number of protons, they discovered that some of these atoms have slightly different masses. They concluded that the variations in mass result, more or less, from the number of neutrons in the nucleus of the atom. Atoms of an element having the same atomic number but different atomic masses get called "isotopes" of that element.
In 1928, Paul Dirac produced equations which predicted an unthinkable thing at the time- a positive charged electron. He did not accept his own theory at the time. In 1932 in experiments with cosmic rays, Carl Anderson discovered the anti-electron, which proved Dirac's equations. Physicists call it the positron.
For each variety of matter there should exist a corresponding 'opposite' or antimatter. Physicists now know that antimatter exists. However, because matter and antimatter annihilates whenever they come in contact, it does not stay around for very long. (By the way, an unsolved problem remains as to why the universe consists of mostly regular matter and not an equal amount of antimatter. Physicsts call this "symmetry breaking".)
There exists not only anti-electrons but in 1955, physicists found the anti-proton, and later the anti-neutron. This allows the existence for anti-atoms, a true form of antimatter.
When scientists found out about the atomic nucleus, they questioned why the positively charged protons should remain so close without repelling. The scientists realized that there must exist new forces at work and the secrets must lie within the nucleus. They knew that the force which holds the protons together must occur much stronger than the electromagnetic force and that the force must act over very small distances (otherwise they would have noticed this force in interactions between the nucleus and the outer electrons).
In 1932, Werner Heisenberg concluded that charged particles bounce photons of light back and forth between them. This exchange of photons provides a way for the electromagnetic forces to act between the particles. The theory says that a proton shoots a photon at the electron, and the electron shoots a photon back at the proton. These photon exchanges go on all the time, very rapidly. However, because no one can see them (measure them), Heisenberg called these exchange particles, virtual photons. (Virtual meaning, not exactly 'real'.)
In 1935 a Japanese physicist, Hideki Yukawa, suggested that exchange forces might also describe the strong force between nucleons. However, virtual photons did not have enough strength for this force, so he thought that there must exist a new kind of virtual particle. Yukawa used Heisenberg's uncertainty principle to explain that a virtual particle could exist for an extremely small fraction of a second. Since its time of existence occurs nearly exactly, there would occur a great uncertainty in the energy of the virtual particle. This uncertainty allowed the particles to exist very strongly only at certain times and the particles could slip in and out of existence. He also calculated that these particles should be about 250 times as heavy as an electron. Later, in 1947, the physicist Cecil F. Powell detected this particle and called it the "pion."
Although the pions describe the transmitters of the strong force, they do not get classed with the other force-transmitting particles, such as the photon or the W and Z particles. Pions now appear not as elementary particles but rather composites made up of "quarks." The strong force gets transmitted by the pions only at relatively larger nuclear levels.
Physicists presently think that all the forces in the universe get carried by some kind of quantum particle. This theory started in 1928 with Paul Dirac stating that photons transmit the electromagnetic force. The theory called "quantum electrodynamics," or QED, developed from work by Richard Feynman, Julian Schwinger, and Sin-Itiro in the late 1940s. The four known forces and their particles appear as follows:
PARTICLE NATURE AND ROLE Photon Carrier of the electromagnetic force (magnetisim, light, heat, EMR, electricity) W+, W-, Z Carrier of the weak force (radioactivity) Gluon (8 types) Carriers of the strong force (holds the quarks) Gravition Carrier of the gravitational force (undetected so far at the time of this writing)
From 1947 until the end of the 1950's, physicists discovered many more new particles (dozens of them). The various types of particles needed a new theory to explain their strange properties.
In 1960, Murray Gell-Mann and Yuval Ne'man independently proposed a method for classifying all the particles then known. The method became known as the Eightfold Way. What the periodic table did for the elements, the Eightfold Way did for the particles. In 1964 Gell-Mann went further and proposed the existence of a new level of elementary particles and called them "quarks" (the spelling derives from a phrase in James Joyce book, Finnegans Wake, "Three quarks for Muster Mark."
Gell-Mann thought there existed at least three types of quarks. They have the names, "up," "down," and "strange." From 1974 thru 1984 the theory predicted three more quarks called "charm," "bottom" (or beauty), and "top" (or truth). And each quark has their corresponding anti-quark.
The theory of the quark explains the existence of several particles including the nucleus of the atom. In fact the proton and neutron each get made up of three quarks and the force which holds the quarks together come from particles called "gluons."
Quarks do not exist by themselves but only in pairs (mesons) or triplets (baryons).
The following charts list the various particle groupings:
LEPTONS (spin 1/2, mass < mesons) NAME MASS LIEFTIME CHARGE SPIN Electron 0.5511 MeV Stable -1 1/2 Positron 0.5511 MeV Stable +1 1/2
105.6 MeV 2 x 10-6 s -1 +1 1/2 1/2
1.78 GeV < 50 eV 291 x 10-15s 0 0 1/2 1/2
Muon neutrino &
0 (?) <.05 MeV Stable (?) 0 0 1/2 1/2
Tau neutrino &
0 (?) <70 MeV Stable (?) 0 0 1/2 1/2
QUARKS (particles with 1/3 or 2/3 charge) NAME MASS LIFETIME CHARGE SPIN
1.5-4.5 MeV Stable * +2/3 -2/3 1/2 1/2
5.0-8.5 MeV Variable * -1/3 +1/3 1/2 1/2
~100 MeV Variable * -1/3 +1/3 1/2 1/2
~1.2 GeV Variable * +2/3 -2/3 1/2 1/2
~4.2 GeV Variable * -1/3 +1/3 1/2 1/2
175 GeV Variable * +2/3 -2/3 1/2 1/2 * As quarks occur only in pairs or triplets, their lifetimes vary
BOSONS (force carrying particles) NAME NATURE MASS LIFETIME CHARGE SPIN Photon Electromagnetic 0 Stable 0 1
Weak force 80.4 GeV 10-25 s +1 -1 1 1 Z Weak force 91.2 GeV 10-25 s 0 1 Gluon Strong force 0 Stable 0 1 Graviton* Gravity 0 Stable 0 2 * Undetected at the time of this writing
MESONS (masses between the electron and proton) NAME MASS LIFETIME CHARGE SPIN Pion (pi-zero) 135 MeV 0.8 x 10-16 s 0 0
140 MeV 2.6 x 10-8 s +1 -1 0 0 Kaon (K-zero) 498 MeV 10-10 s 5 x 10-8 s 0 0
494 MeV 1.2 x 10-8 s 0 0 J/PSI 3.1 GeV 10-20 s 0 1
1.87 GeV 10-12 s 4 x 10-13 s 0 +1 0 UPSILON 9.46 GeV 10-20 s 0 1
BARYONS NAME MASS LIFETIME CHARGE SPIN
938.3 MeV 938.3 MeV Stable (?) Stable (?) +1 -1 1/2 1/2
939.6 MeV 939.6 MeV Stable in nuclei 15 Min. free 0 0 1/2 1/2
1.115 GeV 1.115 GeV 2.6 x 10-10 s 0 0 1/2 1/2
Sigma (sigma +)
Sigma (sigma - )
Sigma (sigma 0 )
1.189 GeV 1.197 GeV 1.192 GeV 0.8 x 10-10 s 1.5 x 10-10 s 6 x 10-20 s +1 -1 0 1/2 1/2 1/2
1.321 GeV 1.315 GeV 1.6 x 10-10 s 3 x 10-10 s -1 0 1/2 1/2 Omega minus 1.672 GeV 0.8 x 10-10 s -1 3/2 Charmed lambda 2.28 GeV 2 x 10-13 s 1 1/2 And this only describes the beginning!
From the time of the ancient Greeks until today, the visual concept of the atom has proved elusive and obscure, yet the mathematical concepts have grown stronger. Although nothing has yet proven absolute, humans can now predict the behavior of atoms with great accuracy. But the world of the atom, the quanta of particles, appears so strange that we can no longer visualize what we think and talk about. The particles have a quality of complete random existence and non-existence about them; and yet the methods of quantum electrodynamics (QED), quantum chromodynamics (QCD), and the whole of quanum mechanics provide such precise, useful, and powerful tools, that it encompasses all of the classical physical laws. The predictions of quantum mechanics have verified themselves many times and to a precision of better than one part in a billion. No predictive method has yet come as close. Even the unproven psychics, soothsayers, and prophets can only dream about such powers of prediction. If you delve into the strange world of atoms, you might start going crazy and start speaking to dogs:
Toto, I've a feeling we're not in Kansas anymore.
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The "Atoms" HyperCard stack (An Apple computer document) got compiled by Jim Walker (alias: Zardoz) of Flight Engineering in September 1988 and transferred to the html language in 1997. This text may change as new information about the atom arrives. If you wish to use this text, please refer to the latest revision.
Bibliography (click on an underlined book title to obtain it):
Particles, by Michael Chester, 1978, Macmillan Publishing
The Particle Explosion, by Frank Close, Michael Marten, Christine Sutton, 1987, Oxford University Press
The Nature of Reality, by Richard Morris, 1987, The Noonday Press
Foundations of College Chemistry, by Morris Hein, 1967, Dickenson Publishing Co., Inc.
The Key to the Universe, by Nigel Calder, 1977, The Viking Press
The Quark and the Jaguar, by Murray Gell-Mann, 1994, W.H. Freeman and Company
The American Heritage Dictionary, Second College Edition
Thanks to Marco Musy from Cern, European Organization for Nuclear Research, for correcting some of the quark, boson, and Tau masses.
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