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Dalton — Thomson — Rutherford — Bohr

5

The Development of the Atomic Model

For more than 2,000 years, the verdict of Aristotelean doctrine had considered Democritus’ atomic model completely useless. It was not until the early 19th century that scientific research found experimental evidence that matter is actually made up of small building blocks.

John Dalton (1766–1844), an English chemist and physicist, started with weather observations and the study of atmospheric phenomena to accurately investigate the chemical properties of gases. In his book, A New System of Chemical Philosophy, published in 1808, Dalton laid the foundation of modern atomic theory:

• All matter is composed of atoms.• Each element consists of identical, undivided atoms. They are responsible for

the characteristics of the element.• In chemical reactions, atoms of different elements combine to form molecules.• In chemical reactions, atoms of one element combine in integer mass ratios

(Dalton’s law of multiple proportions).

Dalton’s model was quickly adopted in the field of chemical research, but it was not precise and specific enough, and it could not explain the electro-physical or electrochemical reaction. Nevertheless, Dalton’s method—to link theory and experiment firmly together— became the standard in physical research. In his honor, the unified atomic mass unit was named a “Dalton” (Da). One Dalton (1 Da) is approximately equal to the mass of 1 proton or 1 neutron. Today, however, it is defined as one-twelfth of the rest mass of an unbound atom of carbon−12 (12C) in its nuclear and electronic ground state, and a new symbol “u” (unified atomic mass unit) has replaced the old label.

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Dalton — Thomson — Rutherford — Bohr | 95

The British physicist Sir Joseph John “J.J.” Thomson (1856–1940) was a profes-sor of experimental physics at the famous Cavendish Laboratory at the University of Cambridge. His predecessors were James Clerk Maxwell (1871–1879) and John William Strutt (Lord Rayleigh, 1879–1884). In the 1890s, Thomson conducted experiments with cathode ray tubes based on similar experiments by the German physicist Johann Wilhelm Hittorf and the English pioneer, Sir William Crookes. In Thomson’s experiments, the cathode ray was deflected by electromagnetic fields; he discovered that the beams had to consist of a single type of negatively charged particles “of bodies much smaller than atoms.”

With Thomson’s discovery it became clear that—in contrast to Dalton’s atomic model—atoms are not indivisible but that they contain small, negatively charged particles. Thomson labeled these particles “corpuscles,” not an unusual description for something that is not yet entirely understood. The “electron,” as it was then called, was the first subatomic elementary particle discovered by man. For this discovery, J.J. Thomson won the 1906 Nobel Prize in Physics.

Together with the doyen of British physics, Lord Kelvin—William Thomson (1824–1907), credited with the Kelvin temperature scale and the formulation of the Second Law of Thermodynamics—formulated a new concept of the atom:

• The negatively charged electrons are tiny particles within the atom.• The positive charge is distributed within the atom.

A cloud, raisin bread, or watermelons are the other possibilities that can be used in imagining Thomson’s atomic model. In this model, the negatively charged electrons are distributed evenly within the atom, the pulp representing the positive charge. Thomson’s model was also dubbed the “plum pudding model.” This model took into account the electrical properties of matter, but it could not explain why elements generated different and unique spectral lines, i.e., why only waves with specific frequencies were sent out (the “fingerprints of the elements”).

Ernest Rutherford (1871–1937) was a physicist born in New Zealand. He finished his postgraduate studies as Thomson’s student at the Cavendish Laboratory in Cambridge. Since Rutherford appeared to be too young for a career in Cambridge, he went to McGill University in Montreal, Canada. At McGill University, he was able to shed light on the mysteries of the newly discovered phenomenon of radioactivity.

Rutherford managed to isolate alpha and beta rays from each other and proved that radioactivity arises from the decay of one element into another. Rutherford was awarded the 1908 Nobel Prize in Chemistry for “investigations into the disintegra-

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96 | CERN: How We Found the Higgs Boson

tion of the elements, and the chemistry of radioactive substances.” Rutherford is often referred to—thanks to his large theoretical and experimental skills—as “the father of nuclear physics.” Einstein called him a “second Newton.”

In 1907, Rutherford went to the University of Manchester, where he performed his most famous experiment (“alpha particle scattering”). In this experiment, Rutherford deflected alpha rays, which were known to be positively charged and therefore had to be made up from the “pudding” in Thomson’s atomic model, from a very thin (0.000004 cm) gold foil. This gold foil was about one thousand atoms thick. Behind that foil there was a circular detector film of zinc sulfide. If the mass of the atom was evenly distributed as in Thomson’s model, all alpha ray particles—consisting of double-charged helium ions—should pass through the film unhindered. In Rutherford’s experiment, however, it turned out that some of the particles were distracted. Some of them were even thrown back.

With regard to his gold foil experiment, Rutherford said, “It was quite the most incredible event that ever happened to me in my life. It was almost as incredible as if you had fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”

In 1911, Rutherford published an article about the results of his scattering experiments, titled “Structure of the Atom.” In that article he described his model of the atom:

• The atom consists of a positively charged, extremely small and extremely solid core.

• Negatively charged electrons move in a circle around this core.• Between the nucleus and electrons there is empty space.

Rutherford calculated that the diameter of the nucleus was less than 3.4 × 10−14 meters. It was known that the gold atom had an overall diameter of about 3 × 10−10 meters. The nucleus therefore was about 10,000 times smaller than the entire atom!

Rutherford developed an atomic model in which the electrons—much like the planets orbiting around the Sun—orbit around the core of the atom. His planetary model superseded Thomson’s pudding model. Rutherford’s model was very clear, but it had one crucial flaw. In his model, the electrons could take any possible orbit around the nucleus. A consequence of this would have been the fact that one single element could have completely different properties. With this model, it was also impossible to explain the fact that different elements had different spectral lines. In addition, the orbiting electrons would—according to the laws

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Dalton — Thomson — Rutherford — Bohr | 97

of electrodynamics—constantly be losing energy and therefore they would crash into the nucleus. Although Rutherford’s atomic model was a huge step forward, it produced unstable elements—which obviously was not right.

Niels Bohr (1885–1962) was one of the most influential physicists of the 20th century. Bohr, aided by Max Planck, Albert Einstein, Werner Heisenberg, and Erwin Schrödinger, put together the foundations of quantum theory, which is the basis of modern atomic physics. Bohr, coming from Danish nobility, had studied in Copenhagen. He then furthered his postdoctoral studies with J.J. Thomson in Cambridge and with Ernest Rutherford in Manchester, respectively. In 1913, Bohr further developed Rutherford’s atomic model by adding concepts from Max Planck’s quantum theory. Bohr’s model did not arise from experiments, but was proven by actual existing properties of the hydrogen atom.

Bohr’s Atomic Model

• Electrons orbit the nucleus exclusively on circular or elliptical orbits.• The electron orbits are located exclusively on certain quantized energy levels.• Transitions between the electron orbits are only possible in jumps (quantum

jumps) caused by a simultaneous absorption or release of energy.

Although Bohr’s atomic model was able to accurately describe the spectral properties of simple atoms such as hydrogen, in more complex atoms, it was not applicable. In 1921, Bohr eventually developed the “structural principle.” It speci-fied the structure of electron trajectories: “the electrons in the atom are arranged in distinctly separate groups, each containing a number of electrons equal to one of the periods in the sequence of the elements, arranged according to increasing atomic number” (Atomic Structure, Nature, March 24, 1921).

In Bohr’s model, the electrons move in certain orbits of fixed, or “quantized,” size and energy, dubbed “shells.” Only the outer shells determine the chemical properties of the atom. Bohr’s model thus provided a theoretical explanation of the chemical elements. It is still valid today—with some modifications applied through the cooperation of Werner Heisenberg—as the basis of modern atomic physics. The 1922 Nobel Prize in Physics was awarded to Niels Bohr “for his services in the investigation of the structure of atoms and of the radiation emanating from them.” In 1921, he founded the Institute for Theoretical Physics in Copenhagen. In the following decades, Bohr’s institute became the center and starting point of international nuclear physics research.

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