Chemistry Reference
In-Depth Information
We will start by constructing the Lewis dot diagram for the molecular
compound known as water, H
2
O. Before we begin, let's refresh our memory
of what the Lewis dot diagrams for oxygen and hydrogen look like.
Notice that the oxygen atom has
two pairs of electrons, and two elec-
trons that are unpaired, or lone,
electrons. These lone electrons rep-
resent bonding sites. The oxygen
atom can do a number of things to
complete its octet. It could steal two
additional electrons and become
the oxide (O
2-
) ion; it could form
two single covalent bonds, sharing
enough electrons to allow it to act
like it has a noble gas configuration
for part of the time; or it could form one double bond, by sharing two pairs
of electrons with a single atom.
When oxygen reacts with hydrogen to form water, each oxygen atom
makes two single bonds with two different hydrogen atoms. Each hydrogen
atom has only a single unpaired electron, so it can only make one single
bond in order to complete its valence shell. Because hydrogen is so small,
each hydrogen atom is considered complete if it obtains the valence con-
figuration of helium, which only has two electrons in its valence shell. Some-
times a hydrogen atom steals an electron to become the hydride (H-) ion,
but in the reaction to form water with oxygen, each hydrogen atom will
form one single bond with an oxygen atom.
Lewis Dot Diagram for Oxygen
and Hydrogen
O
H
Oxygen has 6
valence electrons
Hydrogen has 1
valence electron
Figure 4-3a
In Figure 4-3b you
will see a Lewis dot no-
tation for water. For the
sake of clarity, the elec-
trons that came with the
hydrogen atom are rep-
resented as grey dots,
and the electrons that
came with the oxygen
atom are represented as
black dots.
The lone electron
for each atom acts as a
The Lewis Dot Notation for Water
Each shared pair
of electrons are
also known as a
covalent bond.
By sharing electrons, each element can gain
stability by completing its valence shell. Notice, the
oxygen atom is surrounded by 8 electrons, like the
noble gas neon. Each hydrogen atom is surrounded
by 2 electrons, like the noble gas helium.
Figure 4-3b
