Properties of Atoms, Page 2

Noble Gas Configuration
 The Noble Gas Configuration is the preferred configuration for the arrangement of electrons on all atoms. This configuration imparts a desirable level of stability into a system. Consequently, all atoms will strive to gain or lose electrons in order to create an electronic configuration that is isoelectronic with that of a Noble Gas. Usually, the smaller atomic number elements are able to do this quite easily. The larger atomic number elements are more restricted in their abilities to achieve this condition. Therefore, many of the larger elements must resort to alternate methods of achieving stability.

Oxidation Number
 The Oxidation Number of an atom or group of atoms is the charge that it carries. The term is replacing the older terms of "charge" and "valence."

Pauling Scale

 The Pauling Scale is the most commonly used scale of electronegativity values. The calculations used to arrive at the numbers in the scale are complex. It is most common to simply know the results of those calculations. One of the major benefits of the scale is the simplicity of the numbers. The scale is based on Fluorine having the largest electronegativity with a value of 4.0. The Francium atom is assigned the lowest electronegativity value at 0.7. All other values are located between these extremes. As anchor points, the elements in the second period on the Periodic Chart have values usually rounded off as follows:

Li--1.0 Be--1.5 B--2.0 C--2.5 N--3.0 O--3.5 F--4.0.

Periodic Chart Trends

 The Periodic Chart Trends refer to overall pattern of changes in Properties of Atoms that take place within Groups and Periods on the Periodic Chart. Frequently, it is more useful to know how different elements compare than it is to know specific numerical information. Therefore, the Trends have been developed to aid in doing comparative work. While there will be exceptions, or deviations, in the Trends, the patterns are generally reliable.

The Trends are:

Within a Period from left-to-right:

  • I.P. increases
  • E.A. increases
  • Electronegativity increases
  • Size (radius) decreases

Within a Group from top-to-bottom:

  • I.P. decreases
  • E.A. decrease
  • Electronegativity decreases
  • Size (radius) increases
 

Radius

 The Radius of an atom is a difficult thing to measure. Because of the variable nature of the electrons, the outside edge of an atom is poorly defined. Therefore, the outside edge is not a reliable location for measuring to. As a result, the sizes of atoms are measured in a variety of ways. Among them are terms such as the Atomic Radius and the Covalent Radius. Each will have its own specific definition. Methods such as this will provide relative sizes of atoms that are useful for comparison purposes. Another option is to calculate the Radius of an atom, based on some arbitrary standard. This will not provide an exact size for the atom, but will, again, provide information that is useful for comparisons. This approach calculates the distance between the center of the nucleus and the most probable position of the valence level. The equation used for this purpose is

   
In this equation,

  • n is the value of the valence level.
  • Zeff is the effective nuclear charge on a valence level electron.
  • The term ao is a constant that represents the selection of units for the radius. For instance, if ao is set at 0.54, then the resulting radius is measured in Angstroms.

Stability Factors

 The Stability Factors referred to here are used to determine the numbers of electrons that are gained or lost in chemical bonding processes. As atoms struggle to improve stability by bonding, there are certain methods available to them. Generally, there are four factors that appear most frequently. Listed here, in order of decreasing importance:

  • Desire to achieve a configuration that is isoelectronic with a Noble Gas.
  • Desire to lose all valence level electrons.
  • Desire to lose the valence level p orbital electrons.
  • Desire to maintain filled, empty, and half-filled sets of orbitals.

The desires to lose valence level electrons or valence level p orbital electrons are very similar in terms of preference. In some atoms one of the factors is more important, and for other elements the other factor is more important. Regardless, the idea of half-filling a set of orbitals will provide some stability, but it is definitely a very low priority.

 Questions and comments should be sent to :   kdrews@bcpl.net  

Updated August 1, 2000