5 Steps to a 5 AP Chemistry 2019

Bonding ❮ 159

Again, let us emphasize that the actual structure of the nitrate is not any of the three

shown. Neither is it flipping back and forth among the three. It is an average of all three. All

the bonds are the same, intermediate between single bonds and double bonds in strength

and length.

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 p*2p antibonding 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.

Experiments

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 valence electrons are counted.

STRATEGY

TIP

O O

O

N

−

O O

O

N

−

O O

O

N

−

Figure 11.11 Resonance structures of the nitrate ion, NO 3 -.