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:
Polarity: So now we can predict bonding and shape in representative group molecules (and thus most biomolecules), how about electron density and thus charge distribution? Need two bits of information:
|Molecule||Geometry||Structure||Electronegativities||Bond Dipoles||Molecular Dipole||Model|
||None: two dipoles are of equal magnitude, but opposite in direction and cancel.|
|None: four dipoles are symmetrically arranged to cancel each other out and give a spherically charged but non-polar ion.|
3. Determine Geometry:
- Electronic geometry = Trigonal pyramidal:
- Since there are two lone pairs the trigonal pyramidal electronic geometry give a T-Shape:
4. Determine Polarity:
- Is there a difference in the Electronegativity (EN) of the bonded atoms?
- ENCl = 3.0, ENF = 4.0
- EN = 1.0, there is a difference, and
- The Cl-F bonds are polar, with a small negative charge on each Fluorine.
- One of the bonds is not opposed (the molecule is NOT symmetrical), it is polar.
It turns out that the transfer of an electron from a metal to a non-metal will not generally provide enough energy to favor the process. So how is it that these are in fact favorable reactions?
Let's look at the energy of the process by breaking it into steps and looking at the enthapies of formation starting with free atoms (the reality will be somewhat more complex since we would start with solid metal and molecules, each of which must first react to give free atomic state, but the results are similar). Of course we can get away with this because we are looking at state functions, which as we saw before are pathway independent!
|Ionization Energy||Na Na+ + e-||H = +495 kJ/mol|
|Electron Affinity Energy||Cl + e- Cl-||H = -348 kJ/mol|
|Total||H = + 147 kJ/mol|
|However, this value is for the free ions. If we allow them to come together by coulombic attraction into a crystal lattice a large additional amount of energy is released:|
|Lattice Energy||Na+(g) + Cl-(g) NaCl(s)||H = - 449 kJ/mol|
|Overall||H = - 302 kJ/mol|
|Syllabus / Schedule|
© R A Paselk
Last modified 6 April 2011