## Bonding Supplement

### Exceptions to the "Octet Rule."

 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.

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

Seesaw with angles of 90° & 120° (SF4)

T-shaped with angles of 90° (ClF3)

Linear with angles of 180° (I3-)

Octahedral with angles of 90° (AsF6-)

Tetragonal pyramidal with angles of 90° (ICl5)

Square planar with angles of 90° (XeF4)

# Polarity in Covalent Molecules

 Molecule Geometry Structure Electronegativities Bond Dipoles Molecular Dipole Model Carbon monoxide linear ENC= 2.5, ENO= 3.5 Carbon dioxide linear ENC= 2.5, ENO= 3.5 None: two dipoles are of equal magnitude, but opposite in direction and cancel. Water bent ENH= 2.1, ENO= 3.5 Ammonia trigonal pyramidal ENH= 2.1, ENN= 3.0 Ammonium ion tetrahedral ENH= 2.1, ENN= 3.0 None: four dipoles are symmetrically arranged to cancel each other out and give a spherically charged but non-polar ion.

# Energy of Formation for Ionic Compounds

 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