2.2 Rutherford’s Discovery

The next major development in understanding the atom came from Ernest Rutherford, a physicist from New Zealand who largely spent his scientific career in Canada and England.  He performed a series of experiments using a beam of high-speed, positively charged alpha particles (α particles) that were produced by the radioactive decay of radium; α particles consist of two protons and two neutrons. Rutherford and his colleagues Hans Geiger (later famous for the Geiger counter) and Ernest Marsden aimed a beam of α particles, the source of which was embedded in a lead block to absorb most of the radiation, at a very thin piece of gold foil and examined the resultant scattering of the α particles using a luminescent screen that glowed briefly where hit by an α particle.

What did they discover? Most particles passed right through the foil without being deflected at all.  However, some were diverted slightly, and a very small number were deflected almost straight back toward the source. Rutherford described finding these results: “It was quite the most incredible event that has ever happened to me in my life.  It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you”.

Figure 2.8 Rutherford’s experiment set up

Geiger and Rutherford fired α particles at a piece of gold foil and detected where those particles went, as shown in this schematic diagram of their experiment.  Most of the particles passed straight through the foil, but a few were deflected slightly and a very small number were significantly deflected.

Ref: www.openstax.org/

Here is what Rutherford deduced: Because most of the fast-moving α particles passed through the gold atoms undeflected, they must have traveled through essentially empty space inside the atom.  Alpha particles are positively charged, so deflections arose when they encountered another positive charge (like charges repel each other).  Since like charges repel one another, the few positively charged α particles that changed paths abruptly must have hit, or closely approached, another body that also had a highly concentrated, positive charge.  Since the deflections occurred a small fraction of the time, this charge only occupied a small amount of the space in the gold foil.  Analyzing a series of such experiments in detail, Rutherford drew two conclusions:

• The volume occupied by an atom must consist of a large amount of empty space.
• A small, relatively heavy, positively charged body, the nucleus, must be at the center of each atom.

This analysis led Rutherford to propose a model in which an atom consists of a very small, positively charged nucleus, in which most of the mass of the atom is concentrated, surrounded by the negatively charged electrons, so that the atom is electrically neutral.  After many more experiments, Rutherford also discovered that the nuclei of other elements contain the hydrogen nucleus as a “building block,” and he named this more fundamental particle the proton, the positively charged, subatomic particle found in the nucleus.  With one addition, which you will learn next, this nuclear model of the atom, proposed over a century ago, is still used today.

Figure 2.9. Rutherford’s Experiment

The α particles are deflected only when they collide with or pass close to the much heavier, positively charged gold nucleus. Because the nucleus is very small compared to the size of an atom, very few α particles are deflected. Most pass through the relatively large region occupied by electrons, which are too light to deflect the rapidly moving particles.

Another important finding was the discovery of isotopes.  During the early 1900s, scientists identified several substances that appeared to be new elements, isolating them from radioactive ores.  For example, a “new element” produced by the radioactive decay of thorium was initially given the name mesothorium.  However, a more detailed analysis showed that mesothorium was chemically identical to radium (another decay product), despite having a different atomic mass.  This result, along with similar findings for other elements, led the English chemist Frederick Soddy to realize that an element could have types of atoms with different masses that were chemically indistinguishable. These different types are called isotopes—atoms of the same element that differ in mass. Soddy was awarded the Nobel Prize in Chemistry in 1921 for this discovery.

One puzzle remained: The nucleus was known to contain almost all of the mass of an atom, with the number of protons only providing half, or less, of that mass.  Different proposals were made to explain what constituted the remaining mass, including the existence of neutral particles in the nucleus.  As you might expect, detecting uncharged particles is very challenging, and it was not until 1932 that James Chadwick found evidence of neutrons, uncharged, subatomic particles with a mass approximately the same as that of protons. The existence of the neutron also explained isotopes: They differ in mass because they have different numbers of neutrons, but they are chemically identical because they have the same number of protons. This will be explained in more detail later in this chapter.