Questions for Focused Reading

January 13

13.10

1.    In constructing Lewis Dot Structures (LDS) of  molecules, which electrons are considered and why is it logical that not all electrons be considered?

2.    As a substitute to page 607 in the text, "Summary:  Writing Lewis Structures" consider the following initial steps.

        a.    Place the least electronegative atom in the central position.

        b.    Determine N:  the total number of electrons needed in order for each individual atom to have its preferred number of atoms.

                Determine A:  the actual number of valence electrons available.

                Determine S:  the number of valence electrons that must be shared.

        c.    Distribute the shared valence electrons (S) so as to form bonds between all the atoms.  Two electrons make a bond so the number of bonds must be S/2.  Make multiple bonds if necessary.

3.    The steps identified in question 2 are based on fundamental concepts that need to be understood.

    a.    What is the logic associated with placing the least electronegative atom in the central position?  (Hint:  The atom in the central position will always be required to make the greatest number of bonds.)

    b.    What is meant by the preferred number of valence electrons.

13.12 (read the section on formal charge (starts on p 616) before you read the begining of 13.12)

4.    How do you calculate formal charge?

5.    Formal charge is an excellent way to check if the LDS you've drawn is reasonable.   What must be true about the sum of formal charges in a LDS?

6.    In the box at the bottom of page 619, delete the phrase "for a species containing second-row atoms".  The remaining comment applies to all atoms.

7.    To further emphasize the point being made on page 612, draw the LDS for BeF₂.

8.    If you use the octet rule for SF₆ AND the NAS system we developed in class, how many electrons are to be shared?  The minimum number of bonds that you can have for sulfur hexafluoride is six.  How many electrons are shared when six bonds have to be formed.   (Therefore) How many electrons must be around sulfur?

13.11

9.    According to your text, resonance is invoked when more than one valid LDS can be written for a particular molecule.  I would like to add that in order for two structures to be resonance structures, the only thing different between them must be the placement of electrons.   If you rearrange which atoms are bonded to which, it is no longer a resonance structure.  So here's the approach.  Arrange the atoms.  Add in the electrons.  Check the formal charge.  If there is more than one way to arrange the electrons while retaining acceptable values for formal charge (see question 6 above), then you must draw all the possible resonance structures.

10.    Consider the example of the nitrate ion on page 611.  You may find it useful to think of the nitrate ion not as having two single and one double bond, but rather as having three equivalent bonds, all of them being about 1.3^rd order. [single bonds are first order, double bonds are second order]