Models of the Atom

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Models of the Atom

Overview

Until the early 1900s, the laws of classical physics, established in the seventeenth century by Isaac Newton (1642-1727) and in the nineteenth by James Clerk Maxwell (1831-1879), described the behavior of objects in the everyday world well. But investigations into the structure of the atom began to turn up strange phenomena that the Newtonian picture of the universe could not explain. The succession of atomic models put forth in the first part of the twentieth century reflect the attempts of scientists to understand and predict the weird behavior observed at tiny scales.

Background

It was the Greek philosopher Democritus who in 460 b.c. wondered what the smallest possible particles of matter might be and called them "atoms." At the beginning of the twentieth century, however, no one had yet proved that atoms really existed. All of that changed when a Swiss patent clerk named Albert Einstein (1879-1955) published a theoretical paper that established the existence of atoms once and for all.

In 1827, peering into his microscope, the Scottish botanist Robert Brown (1773-1858) noticed that a grain of pollen floating in a drop of water jiggled about in a random way. In 1905 Einstein developed a mathematical formula for this so-called "Brownian motion," arguing conclusively that it was due to the pollen grains colliding with unseen atoms. His conclusions were confirmed several years later by French physicist Jean Perrin (1870-1942).

Even before Einstein published his landmark paper, an English physicist named Joseph John (J. J.) Thomson (1856-1940) had proposed a model of what the atom might look like. While studying electric discharges in gases, Thomson discovered that rays emanating from a negative source of electricity (a cathode) were composed of negatively charged particles. Thomson concluded from measurements he made that the particles had to be fundamental constituents of the basic building blocks of matter. He called the particles "corpuscles"—later to be known as "electrons." Because atoms are electrically neutral, they had to have parts that were positively charged to balance the negative charge of the electrons. In 1904 Thomson envisaged the atom as a relatively large sphere with the positive charge spread throughout and tiny electrons embedded here and there, like raisins in a pudding.

The next development in unlocking the structure of the atom was an offshoot of French physicist Henri Becquerel's (1852-1908) discovery in 1896 that a uranium coating on a photographic plate emitted rays spontaneously, even in the absence of light. In 1902 Ernest Rutherford (1871-1937), a New Zealander who had worked with Thomson in the 1890s, showed that radioactivity results when an unstable element such as uranium is transformed into another element. Rutherford discovered that this transformation, called "radioactive decay," produced three kinds of particles that he called "alpha," "beta," and "gamma." The beta particles were soon shown to be electrons. Alpha particles, however, turned out to have a mass about four times that of a hydrogen atom and an electric charge twice that of the electron, only positive. Rutherford used the alpha particles to probe the structure of matter. He fired alpha particles at gold beaten into a thin foil. Most of the particles passed through the foil, but some bounced back. This result suggested to Rutherford that the particles were encountering bits of positively charged matter.

More importantly, his findings meant that Thomson's model of diffuse positive charge throughout an atom needed revising (otherwise all the particles would have gone through the foil, not just some). Rutherford concluded that the positive charge in the atom was not spread out as Thomson thought, but concentrated in a tiny part that Rutherford called the nucleus. He imagined that negatively charged electrons occupied a spherical region surrounding the nucleus, orbiting the center as the planets do the sun. The electron sphere is 10,000 times larger than the nucleus, yet almost all the mass of an atom is concentrated in the nucleus.

No sooner had Rutherford proposed his model than it raised a question. Since opposite charges attract, according to Maxwell's equations, orbiting electrons should continuously lose energy, eventually spiraling into the nucleus. Moreover, they should give off a rainbow of colors. Yet neither of these predictions was confirmed by experiments. The answer, when it came, was totally unexpected.

In 1900 the German theoretical physicist Max Planck (1858-1947) was trying to solve a puzzle in physics called the "blackbody problem." The problem stemmed from the inability of Maxwell's theory to explain fully how color changes in objects as they get hot (for example, the way a poker stuck into a fire glows red- or white-hot). Acting out of desperation, Planck proposed that hot bodies do not emit energy in a continuous wave, as classical physics predicts, but instead in discrete lumps or packets called "quanta." Planck announced his solution at a meeting of the Berlin Physical Society in October that same year, but the reception to it was lukewarm. The theory gained some respectability five years later when Einstein used it to explain why, under certain conditions, light shining onto a metal surface in a vacuum can make electrons jump out of the metal.

The young Danish physicist Niels Bohr (1885-1962) introduced Planck's quantum theory into the theory of the atom in 1912. Bohr kept the image of the atom as a miniature solar system but explained that electrons did not spiral out of orbit into the nucleus because, first, they could only orbit at certain allowed distances from the nucleus and, second, they could jump up or down from one energy level to another but could not radiate energy continuously. (Picture the difference between a stone resting on a staircase and one poised on a ramp.) This "quantized" model of the atom also solved the riddle of why chemical elements emit light of only a few colors, a spectrum that is unique for each element. The appeal of Bohr's model was its simplicity, but it could not attack all the problems posed by quantum physics. Bohr continued to tinker with his model to bring it into closer agreement with experimental observations.

In 1927 German theoretical physicist Werner Heisenberg's (1901-1976) well-known contribution, known as the "uncertainty principle," recognized a fixed limit as to how precisely we might hope to know the world of tiny particles. Because any measurement affects what is being measured, we cannot know at any one time both the position and the velocity of a particle. Erwin Schrödinger (1887-1961), an Austrian physicist, further explained that the visual image conjured up by the description of particles as occupying positions and moving with velocities is misleading, since the distinction between wave and particle that we make from our experience in the ordinary world is itself illusory in the realm of subatomic phenomena. The implication of Heisenberg's and Schrödinger's physics to how we might picture the atom was not to try.

It was not until 1932 that the English physicist James Chadwick (1891-1974) discovered the neutron, a neutrally charged particle, proving correct an earlier speculation by Rutherford. Together, neutrons and protons make up the nucleus of an atom. As the middle of the century approached, subsequent investigations into the nature of the atom concentrated on opposite types of matter (for example, anti-electrons, antineutrons, and antiprotons, known collectively as "antimatter") and the forces at work within the nucleus.

Impact

Of such moment was the work, both experimental and theoretical, that answered the question "What are the parts of the atom and how do they fit together?" that every person mentioned in this article—with the exception of Democritus, Newton, Maxwell, and Brown—won a Nobel Prize. (The Nobel Prize began in 1901 and is not awarded posthumously.) For centuries before Thomson discovered the electron, atoms were believed to be the basic building blocks of matter, yet Thomson found a substructure that was 2,000 times lighter than the atom. The discovery of the nucleus was likewise revolutionary because it asked why negatively charged electrons do not fall into the positively charged nucleus, one of many questions about the atom that classical physics could not answer.

In science, theory and observation have to fit. When we observe things happen that we did not expect, the result is a kind of crisis. Quantum mechanics (the name given to the laws that followed from quantum theory) was a solution to the crisis brought on by the failure of classical physics to explain atomic phenomena. The planetary model of the atom provides a sense of mastery and of continuity from the very large to the very small. Imagining atoms as solar systems or as billiard balls smacking into one another has the comfortable feel of the familiar. In providing a model of the atom that is almost impossible to visualize, quantum mechanics takes away that sense of the familiar. The world is thus not so manageable as we had thought.

GISELLE WEISS