For biological purposes, the simplest form of a pure substance that retains all of the properties of that substance is an atom. Atoms are also the simplest form of an element. An element is made up of only one type of atom. For instance, carbon is an element. All carbon atoms look the same and they are the only atoms that are carbon atoms. A different atomic structure would be a different atom and therefore a different element. Consult a periodic table for the complete listing of the 100+ elements (for example, pearl1.lanl.gov/periodic/default.htm). All atoms have three components: protons, neutrons, and electrons. The first element, hydrogen, has one proton, usually one neutron, and one electron. (Hydrogen exists in nature as a combination of three isotopes each of which has a different number of neutrons. Normally the most prominent isotope which has one proton is considered.) All hydrogen atoms for the most part look the same.
Protons are a positive charge and along with neutrons, which carry no charge (neutral), are found in the centrally located nucleus of the atom. Electrons are negatively charged and orbit around the nucleus in a three-dimensional cloud of energy levels. The positive-negative attraction (magnetism) keeps the electrons from flying away from the nucleus. Conversely, the kinetic energy (energy associated with motion) of the electron provides it with the energy of movement and keeps the electron from slamming into the nucleus. The balance between these two forces, magnetism and kinetic energy, defines the orbital pathways of all electrons. If a neutral atom loses or gains one electron (or more), such as might happen during a chemical reaction, the resulting atom has an imbalance between the existing charges. This imbalance creates either an excess positive (+) charge, if the atom lost an electron, or an excess negative (-) charge, if the atom gained an electron. Atoms that have a charge are called ions, and they are very reactive and fundamentally important in chemical reactions.
Excerpted from The Complete Idiot's Guide to Biology © 2004 by Glen E. Moulton, Ed.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.