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Unformatted text preview: a central atom. We have no way of describing the shape of a large,
complex molecule in its entirety all in one step since the number of possible shapes is
overwhelming. What we can do is describe the position of atoms around one single, or central,
Usually the central atom is just that, the central atom to which most of the other atoms are
bonded. For instance, we might choose the carbon in methane, CH4 , or the nitrogen in ammonia,
NH3 . For more complex molecules, the central atom will need to be specified. Consider acetic
acid, CH3 CO2 H. Here we have two carbons, four hydrogens and two oxygens. Even if we
choose an element, we have to decide which one. For instance, if we choose carbon to be the
central element, then we will have to further specify it as the carbon on the left, or the carbon on
the right (or the first or second carbon).
VSEPR mechanics III: Sets
Dakota State University Page 104 of 232 Experiment 9: VSEPR General Chemistry I and II Lab Manual Next, we look for the number of "sets" about the central atom. A set is basically a pair of
electrons, kind of. Hmmm, how to put this. Let's put it in equation form:
# sets = # L.P. + # Bonded atoms
where # L.P. is the number of lone pair, or non-bonding pairs, of electrons assigned to the central
element, and #Bonded atoms is the total number of additional atoms to which the central atom is
attached. The number of lone pairs, or the number of additional atoms bonded to any atom other
than the central atom are not included; the number of sets, or set number, applies only to the
central atom. There are two things that must be made note of.
Note 1: The number of bonded atoms refers to the number of atoms bonded directly to
the central element (only) regardless of the order of the bond (single, double, triple). Even if it
is a multiple bond, all electron pairs must occupy the region of space between the nuclei (to some
extent), so it is irrelevant what type of bond it is.
Note 2: In the rare occasion that we are dealing with the shape of a radical (a molecule
with at least one unpaired electron), each unpaired electron we will count as a lone pair anyway.
This is because even unpaired electrons must exist within an orbital, so an unpaired electron
would occupy the same region of space as a lone pair of electrons.
VSEPR mechanics IV: Molecular Shape
Now we know how many "sets" are around our central element. How do we know its
shape? We look it up! The figures represented in the table below show the mo lecular geometry
based on the set number and on the number of non-bonding pairs of electrons (lone pairs). They
have been determined to be the shape that gets each electron pair as far apart from each other as
possible. Set Number Number of Nonbonding Electron-Pair Geometry Molecular Shape
Dakota State University 0
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