Facts About the Size of Atoms
All matter is composed of atoms; atoms in turn are composed of smaller subatomic particles called protons, neutrons, and electrons. Atoms and subatomic particles are incredibly small--so small, in fact, their size is usually measured in picometers. One picometer is one-trillionth of a meter.
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Types
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There are two different ways to describe the size of an atom--its mass and its radius, where the radius is one-half of the diameter. The mass is given in atomic mass units or daltons, where an atomic mass unit is equal to 1.67 x 10 to the -24 grams. The mass of an atom is approximately 1.67 x 10 to the -24 grams times the number of neutrons and protons in its nucleus. (The mass of an electron is four orders of magnitude smaller than that of a proton or neutron.)
Size
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The electron cloud around an atom doesn't have a definite boundary, so the radius of an atom can't be determined by an exact calculation. The radius, moreover, varies depending on the element and on whether the atom has been ionized (i.e., has lost or gained electrons). As a general rule of thumb, atomic radius typically increases as you go down and to the left across the periodic table. Oxygen, to take one common example, has a radius of only approximately 73 picometers or 73 trillionths of a meter whereas cesium--one of the larger atoms on the periodic table--has a radius of about 265 picometers.
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Considerations
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The nucleus of an atom is tiny in comparison to the size of its electron cloud. According to the University of California San Diego's astronomy tutorial, an atom is so much larger than its nucleus that if the nucleus were the size of a ping-pong ball, its electrons would be a kilometer away.
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References
- Photo Credit ATOM; image by Harald Soehngen from Fotolia.com
