Lewis structures are a simple way of drawing molecules to show the
bonding and "location" of electrons. Despite their simplicity, it turns out that
they are very useful in predicting 3-dimensional structure (Through VSEPR theory.)
and some aspects of reactivity.
The American chemist Gilbert Lewis suggested in 1916 that since noble gas
atoms are very stable, non-noble gas atoms can share electrons with each other
to gain noble gas structure. Atoms sharing two electrons are said to form a
covalent bond between the two atoms.
For example, a hydrogen atom has a single electron. The noble gas helium has
two electrons. By sharing an electron with another hydrogen atom, the two can
form a covalent bond.
Now each hydrogen atom is
surrounded by two electrons, and has the same electronic structure as helium.
In a Lewis structure, individual electrons are shown as dots and covalent
bonds as lines connecting the symbols for the elements. (Multiple lines for
multiple bonds.) Only the valence electrons are shown. For example, the Lewis
diagram for the HF molecule is shown below:
There is a bond
between the hydrogen and fluorine atoms: this is the single line. There are six
electrons in lone pairs around the fluorine atom. The hydrogen atom has two
electrons and thus the same electronic structure as helium, the fluorine atom
has eight total electrons (two from the bond, six in lone pairs.) which is the
same as the noble gas neon.
Some examples of Lewis structures are shown below. Note how each atom has
noble gas structure for the valence electrons