For a modern view of bonding illustrated with QuickTime movies based on quantum calculations you may enjoy the Bonding Supplement.
The importance of molecular shape: recognition at the molecular level in organisms. Shape and electron density are extraordinarily important to the interaction of biomolecules - Examples
VSEPR predicts geometry based on these assumptions in a few simple, sequential, steps:
- Linear with angles of 180° ( a single pair and a triple bond, or two double bonds).
- Trigonal planar with angles of 120° (one double bond and two single pairs).
- Tetrahedral with angles of 109.5° (four single pairs). [model]
These three electron pair geometries can lead to five molecular geometries:
- Linear (carbon dioxide)
- CO_{2}
- valence electrons = 4 + 2x6 = 16
- 6y + 2 = 20, thus 4 fewer electrons than required for all single bonds, 4/2 = 2 multi-bonds (2 double or 1 triple)
- LS: from symmetry C will be central atom, therefore=
- Considering C as the central atom, have 2 bonded atoms and no lone-pairs, therefore
- steric number = 2, so linear electronic geometry, and
- linear molecular geometry
- Trigonal planar (formaldehyde, CH_{2}O)
- formaldehyde, CH_{2}O
- valence electrons = 4 + 2x1 + 6 = 12
- 6y + 2 = 6 x 2 + 2 = 14; so molecule has 2 fewer electrons than required for all single bonds, 1 double bond
- LS: O can only have two bonds, so C will be central atom, therefore =
- Considering C as the central atom, have 3 bonded atoms and no lone-pairs, therefore
- steric number = 3, so trigonal planar electronic geometry, and 3 atoms so
- trigonal planar molecular geometry and a model showing single vs. double bonds:
- Tetrahedral (methane, CH_{4}) [model]
- valence electrons = 4 + 4x1= 8
- four bonds possible, since only 4 pairs, single bonds because only have H's bound to C.
- LS: from symmetry C will be central atom, therefore =
- Considering C as the central atom, have 4 bonded atoms and no lone-pairs, therefore
- steric number = 4, so tetrahedral electronic geometry, and 4 atoms so
- tetrahedral molecular geometry, and rotated for a different view:
- Trigonal pyramidal (ammonia, NH_{3}) [model]
- valence electrons = 5 + 3x1= 8
- only 4 pairs, single bonds because only have H's bound to N, 3 bonds, since only 3 H's
- LS: from symmetry N will be central atom, therefore =
- Considering N as the central atom, have 3 bonded atoms and one lone-pair, therefore
- steric number = 4, so tetrahedral electronic geometry, but only 3 atoms so
- trigonal pyramidal molecular geometry and rotated to view molecule from below:
- Bent (water, H_{2}O) [model]
- valence electrons = 6 + 2x1= 8
- only 4 pairs, single bonds because only have H's bound to O, 2 bonds, since only 2 H's
- LS: from symmetry O will be central atom, therefore =
- Considering O as the central atom, have 2 bonded atoms and 2 lone-pairs, therefore
- steric number = 4, so tetrahedral electronic geometry, but only 2 atoms so
- bent molecular geometry
Exceptions to the "Octet Rule." There are two categories of exceptions to the octet rule among representative elements:
- Small atoms in Periods I & II which cannot accomodate or normally don't accomodate a full set of p-electrons. Thus hydrogen and helium have no ground state p-orbitals and so can only carry a maximum of 2 electrons (a duet). The first three atoms of Period II form a transition between hydrogen (duet) and carbon (octet). Thus lithium, because of its very small size, often forms covalent rather than ionic compounds, with a single bond (e.g. LiH). Similarly beryllium can form compounds with a quartet of electrons around it (e.g. BeH_{2}) and boron can accomodate a sextet (e.g. BH_{3}).
- Atoms in the p-block of Periods 3 and higher can have "expanded valence shells" with 10 or 12 electrons in the outermost shell by using some of their "empty" d-orbitals to hold the extra electrons.
To help determine if the octet rule is followed recall Clark's Method (abbreviated) for determining bonding in covalent Lewis Structures:
- Add up all of the valence electrons in the structure (remember to add one electron for each negative charge, or subtract one for each positive charge)
- If e^{-} = 6y + 2 where y = # atoms other than H, then octet rule is followed with single bonds only.
- If e^{-} < 6y + 2 then probably have multiple bonding with the number of multiple bonds = /2 (remember a triple bond is 2 multiple bonds!). However, note the exceptions below with small atoms (H, Li, Be, and B).
- If e^{-} > 6y + 2 then have an expanded valence shell. Note that if = 2, then pentavalent (10 electrons in the valence shell) , and if = 4, then hexavalent (12 electrons in the valence shell).
- If you can draw more than one structure, then chose the most symmetrical.
- If two or more structures are equally symmetrical, then you probably have resonance and should show all structures connected by double arrows.
Representative atoms with empty d-shells can also have what are sometimes referred to as expanded valence shells. In these cases the d-orbitals also participate in bonding enabling more bonds to be formed. Two additional electronic geometries are possible:
These two electron pair geometries can lead to six new molecular geometries in addition to another way to make a linear molecule:
We will come back and look at these again next time
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© R A Paselk
Last modified 4 March 2011