Introductory Concepts, Page 2

Atom:

The smallest characteristic particle of an element. A single atom will consist of a nucleus made from protons and neutrons, and an electron cloud which will "orbit" around the nucleus. In theory, a single atom of an element will still have all the qualities that are associated with a large quantity of the element. An atom of gold, for example, would go through all of the same chemical processes that a large block of gold would go through.

The Periodic Chart is a complete list of all of the known elements. Each of those elements will exist as individual atoms. The number of protons within the nucleus of the atom will be responsible for determining the type of element that the atom is. That is, An atom with 2 protons will be Helium. An atom with 26 protons will be Iron. An atom with 92 protons will be Uranium. And so on... While atoms of the different elements will prefer to have specific numbers of electrons and neutrons, the quantities of electrons and neutrons can vary radically. In other words, the number of protons for an atom of an element is fixed. The number of electrons and neutrons for an atom of an element is not fixed.

Atomic Mass Unit:

The Atomic Mass Unit, or a.m.u., is an arbitrarily defined unit of mass created by scientists for measuring the masses of atoms and molecules. Because of the very small nature of atoms and molecules, the use of traditional units of mass was awkward. Units such as milligrams, grams, or kilograms created extremely small numbers when measuring the masses of atoms or molecules. The atomic mass unit was designed to eliminate these very awkward numbers. By definition, one atomic mass unit is equal to 1/12th the mass of one atom of Carbon-12.

Based on that definition, one atom of Carbon-12 has a mass of 12.0000 a.m.u. In more traditional measurements, the mass of one Carbon-12 atom would be 1.9927x10-23 grams!! Obviously, the a.m.u. has greatly simplified the process of massing atoms by eliminating the use of some very awkward small numbers. All calculations involving masses of atoms and sub-atomic particles will be based upon this unit of mass. 1 Proton is approximately 1 a.m.u. 1 Neutron is approximately 1 a.m.u. 1 Electron is approximately 0.0005 a.m.u.

Atomic Number:

The Atomic Number is the number of protons that are located in the nucleus of a given atom. Because a fractional part of a proton is never found in a nucleus, the atomic number will always be a whole number. On the Periodic Chart, the atomic number is the whole number that is associated with the symbol of each element. The elements are listed on the Periodic Chart in order of increasing atomic numbers. The atomic number is the single most important factor that determines what an element is and how it will behave. The number of neutrons or electrons found on a given atom can vary without changing the element. If the atomic number, or number of protons changes, then the element itself will change. In radioactive processes, any change in the number of protons is referred to as transmutation. In the process a new element is created. As an example, an atom of iron can commonly have 23, 24 or 26 electrons and still be iron. If the number of protons is changed from 26 to 25 then transmutation has occurred and the new atom manganese is formed. By international agreement, the upper case letter Z is used to represent the atomic numbers of all the elements.

Atomic Unit:

The Atomic Unit, or a.u., is an arbitrarily defined unit of charge. It was originally created to simplify the measurement of charge on ions or sub-atomic particles.		Historically, the earlier units used to measure charge produced a set of very awkward numbers. To eliminate these numbers the, much simpler unit, the atomic unit was created.
		A proton has a charge of +1 atomic units and an electron has a charge of -1 atomic units. Ion charges such as +1, +2, -2, -4, etc. are expressed in atomic units. The a.u. is seldom written next to the numbers, but is understood to be the standard.

Atomic Weight:

Atomic weight, or atomic mass, is the average mass of all the naturally occurring, non-radioactive isotopes of an element. The average is weighted, based on the Percent of Abundance of each of those isotopes. Atomic weight is the decimal number most commonly associated with each element's symbol on the Periodic Chart. The values for the atomic weights of the elements are calculated by summing the products of the different individual Isotope Masses multiplied by the decimal equivalent of their percent of abundance values. Both terms, Isotope Mass and Percent of Abundance, must be given. They can only be determined experimentally, or by looking up the values that a chemist has already determined experimentally.

Example:

(Mass of Isotope 1)x(Percent Abundance/100) + (Mass of Isotope 2)x(Percent Abundance/100) + (Mass of Isotope 3)x(Percent Abundance/100) + +etc. = Atomic Weight

 

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