When atoms are brought into some arrangement, the problem of determining the chemical bonding pattern amounts to figuring out where the electrons are and what they look like. The solution of this problem is known as electronic structure theory and is a quantum mechanical problem. All of chemical bonding can be explained by solving the Schrödinger wave equation, which we will discuss in the next chapter.
A simple model, developed by Lewis in 1916, exists that can be used to gain a qualitative understanding of chemical bonding and electronic structure. This is the Lewis dot model.
The Lewis model involves recognizing that not all electrons in an atom play a role in chemical bonding. Electrons that are closest to the nucleus for a set called the core electrons. These are generally chemically inert and not important for chemical bonding.
The outermost electrons of an atom, called the valence electrons are chemically, important, however, and are treated within the Lewis model.
It is worth noting that for quantitative accuracy, the effects of core electrons does need to be included in some approximate way. This can amount to a particularly difficult problem, especially in heavier elements, where the charge on the nucleus is large, and hence the forces on core electrons cause them to move at speeds near light speed. In that case, a fully relativistic treatment of core electrons is necessary, so if you thought your study of relativity theory in physics was useless, think again.
For elements in groups I-VIII in the periodic table,
Note that the group VIII elements, known as the noble gases, have 8 valence electrons (except He, which has only 2) and are highly non-reactive. We can surmise that 8 valence electrons is a kind of magic number giving special stability to the electronic structure of the atom. We call this an octet, which plays an important role in the Lewis model.
As an example of identifying the number of valence electrons, consider the second period:
For this period, we see that
In the Lewis dot model, valence electrons are represented by dots, while core electrons are not shown explicitly. As an example of this representation, consider the first two periods:
Notice how the dots are drawn as single electrons first before pairs of electrons are drawn. Again, the exception to this is Helium, whose two electrons form a closed valence shell and thus must be drawn as a pair. The element Ne also has a closed valence shell, but with eight electrons drawn as four pairs.
