Lewis Symbols and the Octet Rule
The electrons involved in chemical bonding are the valence electrons, which, for most
atoms, are its outermost electrons. The American chemist, Gilbert Newton Lewis (1875–1946),
suggested a simple way of showing the valence electrons in an atom and tracking them during
bond formation, using what are now known as Lewis symbols.
The Lewis symbol for an element consists of the element’s chemical symbol plus a dot for
each valence electron. For example, sulfur has six valence electrons. Its Lewis symbol is:
The dots are placed on the four sides of the chemical symbol — top, bottom, left, and
right — and each side can accommodate up to two electrons. All four sides are equivalent, which
means that the choice of sides for placing the electrons is arbitrary. However, it is recommended
to spread out the dots as much as possible.
SLP’s Notes!
How do you know the valence electron of a chemical element?
To make an effective Lewis symbol, one must know first the number of valence electrons in a chemical
element. As such, there are two possible ways of determining the valence electron of an element:
(1) Electron Configuration
The electron configuration refers to the distribution of electrons in an atom. Since it reports electron
distribution, you can check which electrons are the outermost. Let’s take sulfur for example. Sulfur
has an electron configuration of:
[Ne] 3s23p4
Here, the outermost electrons are marked in red. In a periodic table, they are usually the last two
sections of the electron configuration. To proceed with the example, 2 + 4 = 6 valence electrons, in
which we saw the Lewis symbol earlier.
(2) Patterns in the Periodic Table
If electron configuration is quite challenging, you might want to take note of patterns in the periodic
table instead. Elements found in the same column generally have the same number of valence
electrons. Refer to the following periodic table. The number indicated is the number of valence
electrons for that group. Columns with no indicated number do not follow this pattern.
1
8
2
3 4 5 6 7
Using this pattern, we can also see that sulfur has 6 valence electrons. Thus, we can now draw its
Lewis symbol.
Atoms often gain, lose, or share electrons to achieve the same number of electrons as the
noble gas closest to them in the periodic table. The noble gases have very stable electron
arrangements, which result to lack of chemical reactivity. Because noble gases generally have
eight valence electrons, many atoms undergoing reactions end up with eight valence electrons.
This observation has led to a guideline known as the octet rule: Atoms tend to gain, lose,
or share electrons until they are surrounded by eight valence electrons. In a Lewis symbol, an
octet is shown as four pairs of valence electrons arranged around the element symbol, as follows:
As such, the octet rule provides a useful framework for introducing many important
concepts of bonding.
Brown, T.L., LeMay, H.E., Bursten, B.E., Murphy, C.J., Woodward, P.M., & Stoltzfus, M.W. (2018).
Chemistry the Central Science. 14th ed. Pearson Education Ltd.