Electron Bookkeeping with Lewis Symbols
Ionic bonds use the principle of electron transfer and
electrostatic attraction to hold together. Some compounds are held
together with covalent bonds which exist because of mutual sharing of
bonding valence electrons. It is useful to be able to keep track of
valence electrons. It is recommended that you use a simple bookkeeping
device called a Lewis symbol, named after their inventor, G.N. Lewis
(1875-1946).
To draw the Lewis symbol for an element, start with the chemical
symbol surrounded by a number of dots (or some other symbol), which
represent the atom's valence electrons. For example, the element
lithium, which has one valence electron in its 2s subshell, has the
Lewis symbol
Li�
In fact, each element in Group IA has a similar Lewis symbol,
because each has only one valence electron. The Lewis symbols for all of
the Group IA elements are
Li� Na� K� Rb� Cs�
The Lewis symbols for the eight A-group elements in period 2 are1
GROUP IA IIA IIIA IVA VA VIA VIIA
O
� �
� .. ..
Symbol Li� �Be� �B� �C� �N: �
: �F: :Ne:
�
� � .. ..
The elements in each group below those given have Lewis symbols
identical to those above except of course for the chemical symbol of the
element. Notice that when an atom has more than four valence electrons,
the additional electrons are shown to be paired with others. Also note
that the group number is also equal to the number of valence electrons.
1. For beryllium, boron, and carbon, the number
of unpaired electrons in the Lewis symbol doesn't agree with the number
predicted from the atom's electron configuration. Boron, for example,
has two electrons paired in its 2s orbital and a third electron in one
of its 2p orbitals; therefore, there is actually only one unpaired
electron in a boron atom. The Lewis symbols are drawn as shown, however,
because when beryllium, boron, and carbon form bonds, they behave
as if they have two, three, and four unpaired electrons, respectively.
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