Lewis Dot Structures Molecules, such as Cl2 and SO2, and multi ...

Lewis Dot Structures
Molecules, such as Cl 2 and SO 2 , and multi-atomic ions, such as NO 3
-
and SO 4
2-
,
are held together by chemical bonds. These bonds involve the sharing of pairs of
electrons between the atoms that are bonded. Electro-static forces that might be
called physical bonds hold together salts, such as NaCl.
Lew dot structures (or models) provide a simplified but very useful picture of
chemical bonding, particularly for molecules or ions formed from atoms in the first
two rows in the periodic table.
Recall that the structure of an atom can be indicated as shown. X is the
symbol for the atom (may be one of two letters), A is the mass number,
and Z is the number of protons in the atomic nucleus. It is also the
number of electrons in the neutral atom.
We can imagine building up a neutral atom by adding Z electrons,
one by one, to the bare nucleus. These electrons go into shells
starting with the one closest to the nucleus. When a particular shell
is filled, additional electrons will go into the shell next furthest from
the nucleus.
A
Z
X
The first shell has room for only two electrons. The second shell has room for
eight electrons, as does the third. These shells actually have two sub-shells. The
eight electrons in these shells are not distributed uniformly, but they are found in
pairs at the corners of a tetrahedron.
Filled shells have exceptional stability. These occur for Z values of 2, 10, and 18;
these are the atomic numbers for the first three rare, or noble, gases, He, Ne,
and Ar. Beyond argon, the shells get larger and can hold more electrons. The
next rare gas is Kr, with Z=36. Electrons in the outer-most shell are called the
valence electrons. They are involved in chemical bonding. The electrons in the
inner shells are called the core electrons.
When a molecule or multi-atomic ion is formed, the component nuclei and
electrons arrange themselves in the most stable configuration. Ideally, each
atom, with its core electrons, will be surrounded by eight valence electrons in the
tetrahedral configuration mentioned above. Bonding between atoms occurs when
electron pairs must be shared. When one electron pair is shared, the resulting
bond is called a single bond. When two pairs are shared, a double bond is
formed. IN this case, the two electron-tetrahedra will share an edge. In a triple
bond, three electron pairs are shared and two tetrahedral share a face. It should
be fairly easy to see that a quadruple bond cannot be formed without abandoning
the tetrahedral electron configuration.

The Lewis Dot Model helps us to understand and predict what these stable
configurations might be. More accurate but more complicated models are
discussed in the chapters on Chemical Bonding.
In the lab work, we will construct a number of Lewis Dot Models to illustrate the
ideas mentioned above and to gain some familiarity with the molecules and ions
we will be studying and working with.
Model as many of the following as you have time for. Do at least four molecules
and four ions. For each, draw your result in the margin and indicate the numbers
of valence electrons and the numbers of single, double, and triple bonds.
N 2 nitrogen
2-
CO 3 carbonate
H 2 O water CN - cyanide
NH 3 ammonia OH - hydroxide
CH 4 methane
-
NO 3 nitrate
C 2 H 4 ethylene
-
NO 2 nitrite
C 2 H 2 acetylene
2-
O 2 peroxide
CO 2 carbon dioxide
3-
PO 4 phosphate
CO carbon monoxide
2-
SO 4 sulfate
SO 2 sulfur dioxide
2-
SO 3 sulfite
BF 3 boron trifluoride SCN - thiocyanate
O 3 ozone
+
NH 4 ammonium
N 2 O nitrous oxide H 3 O + hydronium
NaCl sodium chloride
BeCl 2 beryllium chloride
Notes:
1. Molecules or ions with the same numbers of atoms and valence electrons
have the same structures. This means that the atoms could be rearranged
without changing the bonding or structures. Usually, one of these
rearrangements will be much more stable than the others. The reasons for
this will be discussed in a later chapter.
2. Be considering the tetrahedral configuration for the eight electrons, it is
possible to predict the shapes of these molecules or ions in three
dimensions. Examples: H 2 O is not linear, CO 2 is linear, but SO 2 is not.
NO 3
-
is flat but ClO 3
-
is not.
3. In the Lewis Dot notation, the double bond is shown as X::Y. This does not
imply that one pair is closer to X and the other to Y. The pairs are
horizontal rather than vertical. The double bond is commonly indicated as
X=Y.