Chapter 4, Section 2
The Atomic Nucleus
When subatomic particles were discovered, scientists wondered how these particles were put together in an atom. This was a difficult question to answer, given how tiny atoms are. Most scientists—including J.J. Thomson, the discoverer of the electron—thought it likely that the electrons were evenly distributed throughout an atom filled uniformly with positively charged material. In Thomson’s atomic model, known as the “plum-pudding model,” electrons were stuck into a lump of positive charge, similar to raisins stuck in dough. This model of the atom turned out to be short-lived, however, due to the groundbreaking work of Ernest Rutherford (1871–1937), a former student of Thomson.
Figure 4.6 Born in New Zealand, Ernest Rutherford was awarded the Nobel Prize for Chemistry in 1908. His portrait appears on the New Zealand $100 bill.
Rutherford’s Gold-Foil Experiment
In 1911, Rutherford and his coworkers at the University of Manchester, England, decided to test what was then the current theory of atomic structure. Their test used relatively massive alpha particles, which are helium atoms that have lost their two electrons and have a double positive charge because of the two remaining protons. In the experiment, illustrated in Figure 4.7, a narrow beam of alpha particles was directed at a very thin sheet of gold foil. According to the prevailing theory, the alpha particles should have passed easily through the gold, with only a slight deflection due to the positive charge thought to be spread out in the gold atoms.
Animation 4 Take a look at Rutherford’s gold-foil experiment, its results, and its conclusions.
To everyone’s surprise, the great majority of alpha particles passed straight through the gold atoms, without deflection. Even more surprisingly, a small fraction of the alpha particles bounced off the gold foil at very large angles. Some even bounced straight back toward the source. Rutherford later recollected, “This is almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”
The Rutherford Atomic Model
Based on his experimental results, Rutherford suggested a new theory of the atom. He proposed that the atom is mostly empty space, thus explaining the lack of deflection of most of the alpha particles. He concluded that all the positive charge and almost all the mass are concentrated in a small region that has enough positive charge to account for the great deflection of some of the alpha particles. He called this region the nucleus. The nucleus is the tiny central core of an atom and is composed of protons and neutrons.
Using Inference: The Black Box
Purpose
To determine the shape of a fixed object inside a sealed box without opening the box.
Materials
box containing a regularly shaped object fixed in place and a loose marble
Procedure
Do not open the box.
Manipulate the box so that the marble moves around the fixed object.
Gather data (clues) that describe the movement of the marble.
Sketch a picture of the object in the box, showing its shape, size, and location within the box.
Repeat this activity with a different box containing a different object.
Analysis and Conclusions
Find a classmate who had the same lettered box that you had, and compare your findings.
What experiment that contributed to a better understanding of the atom does this activity remind you of?
The Rutherford atomic model is known as the nuclear atom. In the nuclear atom, the protons and neutrons are located in the nucleus. The electrons are distributed around the nucleus and occupy almost all the volume of the atom. According to this model, the nucleus is tiny compared with the atom as a whole. If an atom were the size of a football stadium, the nucleus would be about the size of a marble.
Although it was an improvement over Thomson’s model of the atom, Rutherford’s model turned out to be incomplete. In Chapter 5, you will learn how the Rutherford atomic model had to be revised in order to explain the chemical properties of elements.