How Much Space Does the Nucleus Take in an Atom?
An atom is the smallest possible component of a chemical element. All substances are made up of different kinds of atoms, arranged in various combinations, and the properties of these substances derive from the properties of the atoms. Atoms are made up of what are called elementary particles: protons, neutrons and electrons. The protons and neutrons are tightly joined at the center in a body called the nucleus, while the electrons orbit the nucleus in a cloud.
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Early Hints
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While the basic idea of atoms as the smallest indivisible units of matter dates to the ancient Greeks, it was not until the 19th century that scientists began to deduce some of their properties. In 1805, the chemist John Dalton surmised that all atoms of a particular element were the same and that they could be joined together to create molecules. In 1897, the physicist J.J. Thomson discovered electrons and correctly guessed that they were contained in all atoms.
Bouncing Back
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Thomson's model of the atom had the negatively charged electrons distributed through a field of positive charge, the so-called "plum pudding" model. In 1909, however, this idea was overturned by a landmark experiment designed by the physicist Ernst Rutherford. His team bombarded a sheet of gold foil with positively charged alpha particles and found that some of them were deflected to a much greater degree than expected--a few even back in the direction from which they came. Thomson's model could not account for this, and Rutherford concluded that the positive charge, as well as most of the mass, had to be concentrated in a very small space in the center of the atom. Rutherford surmised that the electrons orbited the nucleus, much like the planets orbit the sun.
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The Modern Picture
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Rutherford's model of the atom was refined further by Niels Bohr in 1913 to incorporate new discoveries from quantum mechanics. The current model of the atom consists of a number of positively charged protons, combined in the nucleus with some number of neutrons with no electric charge. Surrounding the nucleus in a diffuse cloud are the negatively charged electrons, in equal number to the protons, and carrying very little mass. The key insight is that atoms are mostly empty space. The nucleus contains nearly all of the mass of the atom, and yet its radius is only 1/100,000 of that of the atom. The electrons are far smaller and lighter still. Yet all the properties of the atom arise from these infinitesimal particles.
Consequences
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The fact that atomic nuclei have so much mass and so much positive charge in such a small space creates a number of interesting properties. In particular, because the positively charged protons repel each other, it takes an enormous force to hold them so closely together in the nucleus. Nuclear fission reactions, such as in an atomic bomb or a nuclear power plant, involve breaking these bonds to release the tremendous energy inside the nucleus.
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