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
|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
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.|