Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 109 - General Chemistry - Spring 2013

Lecture Notes 29: 15 April

PREVIOUS

Molecular Geometry, cont.

VSEPR (Valence Shell Electron Pair Repulsion) Theory, cont.

Exceptions to the "Octet Rule." There are two categories of exceptions to the octet rule among representative elements:

  1. 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. BeH2) and boron can accomodate a sextet (e.g. BH3).
  2. 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 Sigma e- = 6y + 2 where y = # atoms other than H, then octet rule is followed with single bonds only.
        • If Sigma e- < 6y + 2 then probably have multiple bonding with the number of multiple bonds = Delta/2 (remember a triple bond is 2 multiple bonds!). However, note the exceptions below with small atoms (H, Li, Be, and B).
        • If Sigma e- > 6y + 2 then have an expanded valence shell. Note that if Delta= 2, then pentavalent (10 electrons in the valence shell) , and if Delta= 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.

Expanded valence shells

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:

  1. Trigonal bipyramidal with angles of 90° & 120° (PCl5)

    ball and stick model of phosphorus pentachloride
  2. Seesaw with angles of 90° & 120° (SF4)

    ball and stick model of sulfur tetrafluoride
  3. T-shaped with angles of 90° (ClF3)

    ball and stick model of Chlorine trifluoride
  4. Linear with angles of 180° (I3-)

    ball and stick model of triodide ion
  5. Octahedral with angles of 90° (AsF6-)

    ball and stick model of Arsenic hexafluoride ion
  6. Tetragonal pyramidal with angles of 90° (ICl5)

    ball and stick model of Iodine pentachloride
  7. Square planar with angles of 90° (XeF4)

    ball and stick model of Xenon tetrafluoride

Polarity in Covalent Molecules

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:

Examples:

Molecule Geometry Structure Electronegativities Bond Dipoles Molecular Dipole Model
Carbon monoxide  linear

  C=O

ENC= 2.5,
ENO= 3.5
 CO structure diagram with dipole arrow  CO structure diagram with dipole arrow  
Carbon dioxide  linear  O=C=O
ENC= 2.5,
ENO= 3.5
 CO2 structure diagram with dipole arrows  None: two dipoles are of equal magnitude, but opposite in direction and cancel.  
 Water  bent
  3D structural diagram of water showing lone pairs of electrons
 water structure diagram

 

ENH= 2.1,
ENO= 3.5
 water structure diagram with bond dipole arrows  water structure diagram with molecular dipole arrow ball and stick model of water with dipolee
 Ammonia  trigonal pyramidal  ammonia structure diagram

 

ENH= 2.1,
ENN= 3.0
 ammonia structure diagram with bond dipole arrows  ammonia structure diagram with molecular dipole arrow ball and stick model of ammonia with dipole
Ammonium ion tetrahedral  ammonium ion structure diagram 

 

ENH= 2.1,
ENN= 3.0
ammonium ion structure diagram with bond dipole arrows  None: four dipoles are symmetrically arranged to cancel each other out and give a spherically charged but non-polar ion. ball and stick model ofammonium ion

Find whether Chlorine trifluoride (ClF3) is polar.

1. Determine Lewis structure:
2. Determine steric number (SN) = 3(bonded atoms) + 2(lone pairs) = 5

3. Determine Geometry:

drawing of trigonal pyramid

4. Determine Polarity:

NEXT


Syllabus / Schedule
home "refractometer" icon
C109 Home
lecture "spectroscope" icon

C109 Lecture Notes

© R A Paselk

Last modified 15 April 2013