Atoms and Elements.

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Atoms are basic components of the matter we are familiar with. Atoms are the smallest part of matter that have chemical properties characteristic of a particular chemical element. Most of the mass of an atom is due to the atomic nucleus. The nucleus consists of protons which have a positve electrical charge and neutrons which have no charge. This is a representation of a carbon atom.

Electrons are small components which are surrounding the nucleus and an electron has a negative charge. An atom with no net electrical charge has the same number of electrons as protons. Electrons are restricted to certain energy levels "shells" and orbitals within those energy levels and these energy levels and orbitals fill in a regular pattern from the lowest energy level outward. Here is an illustration of the orbitals of a carbon atom's outer energy level. Orbital is in some sense a bad word because electrons tend to behave rather strangely in that its orbit is best described as a probability density.

Atomic number and atomic mass. The atomic number of an atom is the number of protons in the atom which, if the atom is neutral, equals the number of electrons. This is important because the number of electrons is related to the chemical properties of the atom. The atomic mass is the total mass of the electrons, protons, neutrons in the atom.

Isotopes are atoms that have the same atomic number(number of protons) but differ in atomic mass. For instance most carbon has six protons and six neutrons and thus atomic mass of of approxiately 12. Some carbon atoms have an atomic mass of 13 so thus have six protons and seven neutrons. All isotopes of the same element have about the same chemical properites because the number pattern of electrons is the same. Many elements have multiple isotopes, some of which may ne radioactive. Radioactive isotopes are called radio-isotopes and are characterised by a constant rate of decay into other elements or isotpes. Some common isotopes in biology and their half lives are here.

The half life of a radio-isotope is the time in which it takes half of a starting amount of radio-isotope to decay into something else. This decay is accompanied by the release of various forms of radiation. This is important for biologists because it allows biologists to use radioisotopes to trace the fate of elements in biological systems and to date fossils by measuring the ratios of different radio-isotopes in the fossil.

pgd revised 8/7/99