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

Modern Atomic Theory
Rutherford’s Atom


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The Aurora Borealis. The colors are due to spectral emissions of nitrogen and oxygen.

The concept of atoms is a very useful one. It explains many important observations, such as why compounds always have the same composition (a specific compound always contains the same types and numbers of atoms) and how chemical reactions occur (they involve a rearrangement of atoms).

Once chemists came to “believe” in atoms, a logical question followed: What are atoms like? What is the structure of an atom? In Chapter 4 we learned to picture the atom with a positively charged nucleus composed of protons and neutrons at its center and electrons moving around the nucleus in a space very large compared to the size of the nucleus.

In this chapter we will look at atomic structure in more detail. In particular, we will develop a picture of the electron arrangements in atoms—a picture that allows us to account for the chemistry of the various elements. Recall from our discussion of the periodic table in Chapter 4 that, although atoms exhibit a great variety of characteristics, certain elements can be grouped together because they behave similarly. For example, fluorine, chlorine, bromine, and iodine (the halogens) show great chemical similarities. Likewise, lithium, sodium, potassium, rubidium, and cesium (the alkali metals) exhibit many similar properties, and the noble gases (helium, neon, argon, krypton, xenon, and radon) are all very nonreactive. Although the members of each of these groups of elements show great similarity within the group, the differences in behavior between groups are striking. In this chapter we will see that it is the way the electrons are arranged in various atoms that accounts for these facts.


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Neon sign for a Chinese restaurant in New York City.

Rutherford’s Atom


· To describe Rutherford’s model of the atom.

Remember that in Chapter 4 we discussed the idea that an atom has a small positive core (called the nucleus) with negatively charged electrons moving around the nucleus in some way (Fig. 11.1). This concept of a nuclear atom resulted from Ernest Rutherford’s experiments in which he bombarded metal foil with particles (see Section 4.5). Rutherford and his coworkers were able to show that the nucleus of the atom is composed of positively charged particles called protons and neutral particles called neutrons. Rutherford also found that the nucleus is apparently very small compared to the size of the entire atom. The electrons account for the rest of the atom.

Figure 11.1.An illustration shows a gray sphere with electrons, represented by pink spheres labeled with minus sign to represent the negative charge are scattered around a nucleus, represented by blue sphere labeled with plus sign to represent the positive charge.

The Rutherford atom. The nuclear charge is balanced by the presence of electrons moving in some way around the nucleus.

A major question left unanswered by Rutherford’s work was, What are the electrons doing? That is, how are the electrons arranged and how do they move? Rutherford suggested that electrons might revolve around the nucleus like the planets revolve around the sun in our solar system. He couldn’t explain, however, why the negative electrons aren’t attracted into the positive nucleus, causing the atom to collapse.

At this point it became clear that more observations of the properties of atoms were needed to understand the structure of the atom more fully. To help us understand these observations, we need to discuss the nature of light and how it transmits energy.