The word atom is derived from the Greek word atom which means indivisible. The Greeks concluded that matter could be broken down into particles too small to be seen. These particles were called atoms. Atoms are composed of three types of particles: protons, neutrons, and electron. Protons and neutrons are responsible for most of the atomic mass. Example; if a person weighed 150 lbs. approximately 149 lbs. and 15 ounces of that person would be comprised of protons and neutrons while only 1 ounce of their being would be made of electrons. Both the protons and neutrons reside within the nucleus. Protons have a positive charge, neutrons have no charge; they are neutral. Electrons reside in orbitals around the nucleus and have a negative charge. Atomic number is derived from the number of protons in an atom. Example, Helium, (H) = 1. The number of protons in an element is constant (e.g., H=1, Ur=92) but neutron number may vary, so mass number (protons + neutrons) may vary. However, the same element may contain varying numbers of neutrons; these forms of an element are called isotopes. The chemical properties of isotopes are the same, although the physical properties of some isotopes may be different. Some isotopes are radioactive-meaning they "radiate" energy as they decay to a more stable form, perhaps another element half-life: time required for half of the atoms of an element to decay into stable form. Another example is oxygen; with atomic number of 8 can have 8, 9, or 10 neutrons.
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Simon, Eric J., Jane B. Reece, Jean Dickey, and Neil A. Campbell. Campbell Essential Biology. San Francisco: Benjamin Cummings, 2010. Print, (page 25). Structure of an Atom: http://www.nyu.edu/pages/mathmol/textbook/atoms.html