Inside the atom

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Figure 1 Every atom consists of a nucleus surrounded by electrons.
Figure 1 Every atom consists of a nucleus surrounded by electrons.

You can picture the nucleus of an atom as a minuscule cluster of particles surrounded by a whirling cloud of much tinier particles, the electrons. The nucleus itself is composed of neutrons and protons, collectively known as nucleons. Protons are positively charged, whereas neutrons, as the name implies, carry no charge. It is the number of protons that gives the element its chemical identity; in other words, no two different elements have the same number of protons (Figure 1). Although the nucleus contains most of the mass of the atom (about 99%), the volume it occupies is very small, leaving a relatively large empty space between the nucleus and the electrons.

Figure 2 Uranium-235.
Figure 2 Uranium-235.

Hydrogen, the lightest of all elements, has only a single proton. Helium has two. Carbon, the basis of all life and the main constituent of fossil fuels, contains 6 protons. Oxygen has 8. The Uranium atom, with 92 protons, is the heaviest of all natural elements that make up our planet. Besides these natural elements, other elements have been discovered, but only in a laboratory environment.

Every atom is designated by its atomic number (P), which represents its number of protons and by its atomic mass (A), which is equal to the total number of neutrons (N) and protons AP( X). Almost all uranium naturally present in the universe have 92 protons (and also 92 electrons)and 146 neutrons (P = 92, N = 146, and A = 238) and is represented as or simply 238U. The nucleus of a small fraction, however, has three less neutrons (but still 92 protons and 92 electrons); as it will be seen, this form of uranium called uranium-235(235U) is very unstable and plays an important role in construction of both nuclear reactors and nuclear bombs(Figure 2).

Different atoms make up everything in the universe. A list of these elements along with their atomic masses and numbers are summarized in the Periodic Table shown on the next page.


(1) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005

Further Reading

Bodansky, Nuclear Energy Principles, Practices, and Prospects, Second Ed., Springer, 2004.

Seaborg, G., T., Peaceful Uses of Nuclear Energy, University Press of the Pacific, 2005.

International Journal of Nuclear Engineering and Design, Direct Science Elsevier Publishing Company, devoted to the Thermal, Mechanical, Material and Structural Aspects of Nuclear Fission.

Journal of Fusion Energy, Springer Netherlands. It features articles pertinent to development of thermonuclear fusion.

External Links

Federation of American Scientists (

International Atomic Energy Agency (

DoE Office of Nuclear Energy, Science & Technology (

American Nuclear Society, (

World Association of Nuclear Operator (WANO) (