Introductory Chemistry: A Foundation - Zumdahl S.S., DeCoste D.J. 2019

Chemical Foundations: Elements, Atoms, and Ions
The Structure of the Atom


· To learn about the internal parts of an atom.

· To understand Rutherford’s experiment to characterize the atom’s structure.

Dalton’s atomic theory, proposed in about 1808, provided such a convincing explanation for the composition of compounds that it became generally accepted. Scientists came to believe that elements consist of atoms and that compounds are a specific collection of atoms bound together in some way. But what is an atom like? It might be a tiny ball of matter that is the same throughout with no internal structure—like a ball bearing. Or the atom might be composed of parts—it might be made up of a number of subatomic particles. But if the atom contains parts, there should be some way to break up the atom into its components.

Many scientists pondered the nature of the atom during the 1800s, but it was not until almost 1900 that convincing evidence became available that the atom has a number of different parts.

A physicist in England named J. J. Thomson showed in the late 1890s that the atoms of any element can be made to emit tiny negative particles. (He knew the particles had a negative charge because he could show that they were repelled by the negative part of an electric field.) Thus he concluded that all types of atoms must contain these negative particles, which are now called electrons .

On the basis of his results, Thomson wondered what an atom must be like. Although he knew that atoms contain these tiny negative particles, he also knew that whole atoms are not negatively or positively charged. Thus he concluded that the atom must also contain positive particles that balance exactly the negative charge carried by the electrons, giving the atom a zero overall charge.

Another scientist pondering the structure of the atom was William Thomson (better known as Lord Kelvin and no relation to J. J. Thomson). Lord Kelvin got the idea (which might have occurred to him during dinner) that the atom might be something like plum pudding (a pudding with raisins randomly distributed throughout). Kelvin reasoned that the atom might be thought of as a uniform “pudding” of positive charge with enough negative electrons scattered within to counterbalance that positive charge (Fig. 4.3). Thus the plum pudding model of the atom came into being.

Figure 4.3.An illustration representing the plum pudding model of the atom shows a spherical cloud of positive charge, represented by a blue sphere, with electrons, represented by pink spheres, dispersed throughout. A photo shows a small cake with raisins and nuts interspersed throughout it.

Photo by bit245/

One of the early models of the atom was the plum pudding model, in which the electrons were pictured as embedded in a positively charged spherical cloud, much as raisins are distributed in an old-fashioned plum pudding.

If you had taken this course in 1910, the plum pudding model would have been the only picture of the atom described. However, our ideas about the atom were changed dramatically in 1911 by a physicist named Ernest Rutherford (Fig. 4.4), who learned physics in J. J. Thomson’s laboratory in the late 1890s. By 1911 Rutherford had become a distinguished scientist with many important discoveries to his credit. One of his main areas of interest involved alpha particles ( particles), positively charged particles with a mass approximately times that of an electron. In studying the flight of these particles through air, Rutherford found that some of the particles were deflected by something in the air. Puzzled by this, he designed an experiment that involved directing particles toward a thin metal foil. Surrounding the foil was a detector coated with a substance that produced tiny flashes wherever it was hit by an particle (Fig. 4.5). The results of the experiment were very different from those Rutherford anticipated. Although most of the particles passed straight through the foil, some of the particles were deflected at large angles, as shown in Fig. 4.5, and some were reflected backward.

Figure 4.4.The Structure of the Atom

Library of Congress

Ernest Rutherford (1871—1937) was born on a farm in New Zealand. In 1895 he placed second in a scholarship competition to attend Cambridge University but was awarded the scholarship when the winner decided to stay home and get married. Rutherford was an intense, hard-driving person who became a master at designing just the right experiment to test a given idea. He was awarded the Nobel Prize in chemistry in 1908.

Figure 4.5.An illustration shows a rectangular block containing an alpha-particle emitting source. A beam of alpha particles is directed from it toward a square piece of gold foil, from which some particles scatter in different directions but most pass straight through. There is a circular screen surrounding the foil to detect the scattered particles with a slit to allow the beam to pass through.

Rutherford’s experiment on -particle bombardment of metal foil.

This outcome was a great surprise to Rutherford. (He described this result as comparable to shooting a gun at a piece of paper and having the bullet bounce back.) Rutherford knew that if the plum pudding model of the atom was correct, the massive particles would crash through the thin foil like cannonballs through paper [as shown in Fig. 4.6(a)]. So he expected the particles to travel through the foil experiencing, at most, very minor deflections of their paths.

Figure 4.6.A set of two illustrations show the expected and actual results of the metal foil experiment. In the first illustration, electrons, represented by red spheres labeled with a minus sign to represent the negative charge, are scattered throughout a spherical blue cloud of diffuse positive charge. This image shows the expected results of Rutherford’s metal foil experiment demonstrating that if Thomson’s plum pudding model were correct, the alpha particles would travel through the foil with very minor deflections in their paths. In the second illustration, electrons, represented by red spheres labeled with a minus sign to represent the negative charge, are scattered throughout a spherical grey area. At the center is the concentrated positive charge represented by a small blue sphere labeled n plus. This image demonstrates the actual results of Rutherford’s metal foil experiment, showing that most alpha particles passed straight through, and with a few deflected at large angles.

Rutherford concluded from these results that the plum pudding model for the atom could not be correct. The large deflections of the particles could be caused only by a center of concentrated positive charge that would repel the positively charged particles, as illustrated in Fig. 4.6(b). Most of the particles passed directly through the foil because the atom is mostly open space. The deflected particles were those that had a “close encounter” with the positive center of the atom, and the few reflected particles were those that scored a “direct hit” on the positive center. In Rutherford’s mind these results could be explained only in terms of a nuclear atom —an atom with a dense center of positive charge (the nucleus ) around which tiny electrons moved in a space that was otherwise empty.

Critical Thinking

· The average diameter of an atom is m. What if the average diameter of an atom were cm? How tall would you be?

He concluded that the nucleus must have a positive charge to balance the negative charge of the electrons and that it must be small and dense. What was it made of? By 1919 Rutherford concluded that the nucleus of an atom contained what he called protons. A proton has the same magnitude (size) of charge as the electron, but its charge is positive. We say that the proton has a charge of and the electron a charge of .

Rutherford reasoned that the hydrogen atom has a single proton at its center and one electron moving through space at a relatively large distance from the proton (the hydrogen nucleus). He also reasoned that other atoms must have nuclei (the plural of nucleus) composed of many protons bound together somehow. In addition, Rutherford and a coworker, James Chadwick, were able to show in 1932 that most nuclei also contain a neutral particle that they named the neutron . A neutron is slightly more massive than a proton but has no charge.

Critical Thinking

· You have learned about three different models of the atom: Dalton’s model, Thomson’s model, and Rutherford’s model. What if Dalton was correct? What would Rutherford have expected from his experiments with the metal foil? What if Thomson was correct? What would Rutherford have expected from his experiments with the metal foil?