Lewis Diagrams

Lewis used simple diagrams (now called Lewis diagrams) to keep track of how many electrons were present in the outermost, or valence, shell of a given atom. The kernel of the atom, i.e., the nucleusThe collection of protons and neutrons at the center of an atom that contains nearly all of the atoms's mass. together with the inner electrons, is represented by the chemical symbolA one- or two-letter abbreviation for an individual element., and only the valence electrons are drawn as dots surrounding the chemical symbol. Thus the three atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. shown in Fig. 1 from Electrons and Valence can be represented by the following Lewis diagrams:

If the atom is a noble-gasA state of matter in which a substance occupies the full volume of its container and changes shape to match the shape of the container. In a gas the distance between particles is much greater than the diameters of the particles themselves; hence the distances between particles can change as necessary so that the matter uniformly occupies its container. atom, two alternative procedures are possible. Either we can consider the atom to have zero valence electrons or we can regard the outermost filled shell as the valence shell. The first three noble gases can thus be written as

EXAMPLE 1 Draw Lewis diagrams for an atom of each of the following elements:

Li N F Na

SolutionA mixture of one or more substances dissolved in a solvent to give a homogeneous mixture. We find from the periodic tableA chart showing the symbols of the elements arranged in order by atomic number and having chemically related elements appearing in columns. inside the front cover that Li has an atomic numberThe number of protons in the nucleus of an atom; used to define the position of an element in the periodic table; represented by the letter Z. of 3. It thus contains three electrons, one more than the noble gas He. This means that the outermost, or valence, shell contains only one electron, and the Lewis diagram is

Following the same reasoning, N has seven electrons, five more than He, while F has nine electrons, seven more than He, giving

Na has nine more electrons than He, but eight of them are in the kernel, corresponding to the eight electrons in the outermost shell of Ne. Since Na has only 1 more electron than Ne, its Lewis diagram is

Notice from the preceding example that the Lewis diagrams of the alkali metals are identical except for their chemical symbols. This agrees nicely with the very similar chemical behavior of the alkali metals. Similarly, Lewis diagrams for all elements in other groups, such as the alkaline earths or halogens, look the same. The Lewis diagrams may also be used to predict the valences of the elements. Lewis suggested that the number of valences of an atom was equal to the number of electrons in its valence shell or to the number of electrons which would have to be added to the valence shell to achieve the electronic shell structure of the next noble gas. As an example of this idea, consider the elements Be and O. Their Lewis diagrams and those of the noble gases He and Ne are

Comparing Be with He, we see that the former has two more electrons and therefore should have a valence of 2. The element O might be expected to have a valence of 6 or a valence of 2 since it has six valence electrons—two less than Ne. Using rules of valence developed in this way, Lewis was able to account for the regular increase and decrease in the subscripts of the compounds in the table found in the Valence section, and reproduced here. In addition he was able to account for more than 50 percent of the formulas in the table. (Those that agree with his ideas are shaded in color or gray in the table. You may wish to refer to that table now and verify that some of the indicated formulas follow Lewis’ rules.) Lewis’ success in this connection gave a clear indication that electrons were the most important factor in holding atoms together when molecules formed.

Despite these successes, there are also difficulties to be found in Lewis’ theories, in particular for elements beyond calcium in the periodic table. The element Br (Z = 35), for example, has 17 more electrons than the noble-gas Ar (Z = 18). This leads us to conclude that Br has 17 valence electrons, which makes it awkward to explain why Br resembles Cl and F so closely even though these two atoms have only seven valence electrons.