5 Steps to a 5 AP Chemistry

Bond Length, Strength, and Magnetic Properties

The length and strength of a covalent bond is related to its bond order. The greater

the bond order, the shorter and stronger the bond. Diatomic nitrogen, for example, has

a short, extremely strong bond due to its nitrogen-to-nitrogen triple bond.

One of the advantages of the molecular orbital model is that it can predict some of the

magnetic properties of molecules. If molecules are placed in a strong magnetic field, they

exhibit one of two magnetic behaviors—attraction or repulsion. Paramagnetism, the

attraction to a magnetic field, is due to the presence of unpaired electrons; diamagnetism,

the slight repulsion from a magnetic field, is due to the presence of only paired electrons.

Look at Figure 11.10, the MO diagram for diatomic oxygen. Note that it does have two

unpaired electrons in the π*2pantibonding orbitals. Thus one would predict, based on the

MO model, that oxygen should be paramagnetic, and that is exactly what is observed in the

laboratory.

Experimental

There have been no experimental questions concerning this material on recent AP

Chemistry exams.

Common Mistakes to Avoid

1. Remember that metals +nonmetals form ionic bonds, while the reaction of two non-
   metals forms a covalent bond.


2. The octet rule does not always work, but for the representative elements it works the
   majority of the time.


3. Atoms that lose electrons form cations; atoms that gain electrons form anions.


4. In writing the formulas of ionic compounds, make sure the subscripts are in the lowest
   ratio of whole numbers.


5. When using the crisscross rule be sure the subscripts are reduced to the lowest whole-
   number ratio.


6. When using the N − A =S rule in writing Lewis structures, be sure you add electrons
   to the A term for a polyatomic anion, and subtract electrons for a polyatomic cation.


7. In the N − A =S rule, only the valenceelectrons are counted.


8. In using the VSEPR theory, when going from the electron-group geometry to the
   molecular geometry, start with the electron-group geometry; make the nonbonding
   electrons mentally invisible; and then describe what is left.


9. When adding electrons to the molecular orbitals, remember: lowest energy first. On
   orbitals with equal energies, half fill and then pair up.


10. When writing Lewis structures of polyatomic ions, don’t forget to show the charge.


11. When you draw resonance structures, you can move only electrons (bonds). Never
    move the atoms.


12. When answering questions, the stability of the noble-gas configurations is a result, not
    an explanation. Your answers will require an explanation, i.e., lower energy state.

158  Step 4. Review the Knowledge You Need to Score High

STRATEGY

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