Background to Rutherford’s Experiment

Background to Rutherford’s Experiment

To understand the context of Rutherford’s famous scattering experiment, two developments in the physics of the 1800s to early 1900s need to be considered. One development is primarily theoretical—the nature of the atom—and the other primarily empirical—the discovery of radioactivity.

Evolution of the Nature of the Atom

Theories, meaning non-observable ideas, are created to describe and explain some known evidence. Describing and explaining are the first and easiest criteria for a theory to meet. The most difficult test of a theory is its ability to generate testable predictions that are not falsified by subsequent experiments (for a more complete discussion, see the CRYSTAL-Alberta site).

The Dalton Atomic Model

John Dalton created his theory of atoms to explain three important scientific laws: the empirical laws of definite composition, multiple proportions, and conservation of mass. His key idea, published in the early 1800s, is that matter is composed of indestructible, indivisible atoms that are identical for one element, but different for other elements. The model most often associated with the Dalton theory is like a billiard ball.

The Thomson Atomic Model

J. J. Thomson used the experimental work of Arrhenius on the electrical nature of solutions, the quantitative work of Faraday with electricity and solutions, and his own key experiment on the charge-to-mass ratio of cathode rays to propose that the atom is not indivisible, falsifying the Dalton model. According to Thomson, the atom is composed of negatively charged electrons within a positively charged sphere. This idea, first reported in 1899, is often represented as the “raisin-bun” model.

The Nagaoka Atomic Model

Around the same time that Thomson was discussing his atomic model, Hantaro Nagaoka proposed a “Saturnian model” of the atom, in which the electrons orbit around a central positively charged body much like the rings around the planet Saturn. Nagaoka used this idea in an attempt to explain the regularity of spectral lines of the elements.

Radioactivity

The discovery of radioactivity is one of many examples in science of serendipity—the accidental discovery of something new, especially while looking for something entirely different. Given human nature, unexpected events are sometimes discarded, but scientists with open minds welcome these unexpected events. As Louis Pasteur famously said, “in the fields of observation chance favors only the prepared mind”.

Becquerel’s Discovery of Radioactivity

Henri Becquerel, like his father and grandfather, studied phosphorescence—the absorption of light by a substance and the re-emission of light for a period of time thereafter. Cathode rays were a very popular research topic and Becquerel sought a connection with his work. He hypothesized that the glow in cathode ray tubes might relate to phosphorescent minerals. Quite accidentally, in 1896, he stored some photographic plates, wrapped in thick paper, along with a piece of an uranium compound in a drawer while waiting for the weather to clear for his phosphorescent studies. He was surprised to find that the photographic plate was exposed showing the outline of the piece of uranium compound. Intrigued by this, he did many detailed tests and quickly confirmed that some unknown kind of radiation was emitted by the uranium compound, straight through the paper, and onto the photographic plate. Becquerel recognized this as something new but did not know what this meant.

Empirical Studies of Radioactivity

Immediately following Becquerel’s discovery, a systematic investigation of this new phenomenon was undertaken primarily by Marie Curie and others including her husband Pierre. Marie Curie’s careful empirical work, first reported in 1898, is a model example of the creation of scientific knowledge by inductive reasoning (see the CRYSTAL-Alberta site). Not only were many empirical properties of radioactivity established, but also the new elements polonium and radium were discovered.

Rutherford’s Studies of Radioactivity

Neither Marie Curie nor anyone else had any idea about the nature of radioactivity or its cause. More systematic collection and organization of observations would be required. Some of this work was carried out by Ernest Rutherford as a student of J. J. Thomson at Cambridge University from 1895 to 1898. By careful experimentation, Rutherford discovered alpha and beta rays but still had little idea of their nature or origin.

In 1898, Rutherford became a physics professor at McGill University and teamed up with the young chemist Frederick Soddy. Together they showed that radioactive decay is the transformation of one element into another. Rutherford also studied alpha particles and eventually determined that alpha particles were charged helium atoms. Becquerel was the first to show that beta particles were electrons.
In 1907, Rutherford returned to England and, together with Hans Geiger, invented specially prepared screens of zinc sulfide (scintillation screens) that provided a unique optical method of counting alpha particles. This invention was to prove very useful as Rutherford changed the focus of his research from studies of radioactivity to a study of the distribution of mass and charge within the atom.