(2) Molecular Geometry
, Chapters 8.5-7; 9.1-3, 5, pp. 317-328; 344-360, 361-367.
(Ch 8) 34, 46, 50, 54, 56, 81, 86; (Ch 9) 21, 22, 24, 26, 36
(a) Lewis Structures
allow chemists to write molecular structures in a way that identifies areas of
chemical reactivity and electronic behavior.
They are useful for main-group covalent molecules
and solids, but are not viable for metallic systems.
Such structures can also give predictions of
bond distances and bond angles.
Rules for Writing Lewis Structures
Create the skeleton
showing how the atoms will be connected to give the final molecular
As a rule of thumb, the most electronegative elements are found on the
extremities of molecules, the more electropositive elements are found on the interior.
Thus, F, Cl, O are typically on the outer regions, B, C, N, Si, P, etc. are in the interior.
(with just one valence AO) is also found on the exterior.
2. Count the total number of valence electron pairs
, taking into account the overall net
charge of the molecule.
The number of valence electrons contributed by an atom
corresponds to its group number, e.g., B has 3 electrons, C has 4 electrons, N has 5
electrons, O has 6 electrons, etc.
The number of pairs is the total number of valence
electrons divided by two.
Draw a single covalent bond
between each pair of atoms that are connected; this utilizes
one electron pair for each line drawn.
4. Complete an octet of electrons
around the “terminal atoms” (those at the extremities of
the molecule) by placing
(nonbonding orbitals assigned to a single atom);
utilize multiple bonds if the inner atoms do not establish an octet of electrons by shifting
lone pairs or using the remaining pair of electrons; if there are several possible Lewis
structures, then see if
can be established.
at each element:
= # valence e
# valence e
at atom in Lewis structure.
To count the # of valence electrons around the element in the Lewis structure, a
counts as two
counts as one
(the electrons in a bond pair are equally shared
by the two atoms).
NOTE: the sum of all formal charges in the Lewis structure equals
the total charge on the molecule.
The isocyanate ion, OCN
, is an important ion in polymer chemistry.
What is its Lewis
The number of valence electrons is (6 + 4 + 5 + 1) = 16 valence electrons or
The following Lewis structures are possible, each of which satisfies the
octet rule at every atom:
We can calculate the formal charges for each Lewis structure: