10.21. Best Lewis Formula and Molecular Geometry
A student writes the Lewis electron-dot formula for the carbonate anion, CO32âˆ’, as
(link to the picture)
a. Does this Lewis formula obey the octet rule? Explain. What are the formal charges on the atoms? Try describing the bonding for this formula in valence bond terms. Do you have any difficulty doing this?
b. Does this Lewis formula give a reasonable description of the electron structure, or is there a better one? If there is a better Lewis formula, write it down and explain why it is better.
c. The same student writes the following resonance description for CO2: Is there something wrong with this description? (What would you predict as the geometries of these formulas?)
d. Is one or the other formula a better description? Could a value for the dipole moment help you decide?
e. Can you write a Lewis formula that gives an even better description of CO2? Explain your answer.
10.22. Molecular Geometry and Bonding
Draw four different Lewis electron-dot formulas for the SeO2 molecule. (HINT: One should have two single bonds, two have a single and a double bond, and one two double bonds).
a. Are there any electron-dot formulas that you expect to give the best description? How would you describe the bonding in terms of electron-dot formulas?
b. Describe the bonding in SeO2 in valence bond terms. If there is delocalized bonding, note this in your description and explain how molecular orbital theory can be useful here.
c. Determine the arrangement of electron pairs about Se in the SeO2 molecule. What would you expect for the molecular geometry of the SeO2 molecule?
d. Would you expect the Oâ€”Seâ€”O angle in the SeO2 molecule to be greater than, equal to, or less than 120Â°. Explain your answer.
e. Draw an electron-dot formula for the H2Se molecule and determine its molecular geometry.
f. Compare the Hâ€”Seâ€”H bond angle to the Oâ€”Seâ€”O bond angle. Is it larger or smaller? Explain.
g. Determine whether the H2Se and SeO2 molecules have dipole moments. Describe how you arrived at your answer.
no it doesn't obey the octet rule. Look at the carbon is has too many electrons around it! each bond is 2 electrons and it has 5 bonds - big no no!
To assign charge you count the number of electrons around an atom as drawn. For each bond to assign one electron to each atom so carbon has 5 electrons - which is one more than the 4 valance electrons that neutral C has. So C has a formal negative charge from that structure.
The top oxygen is neutral, and the left oxygen is neutral (each 6 electrons) but the right one has 7 electrons so is negative.
The bonding in VB terms: just try pushing arrows to draw resonance structures and see how that goes. I can't really describe it. But it'll be weird from the structure.
The better one is one that's symmetrical which each oxygen atom being the same (make the molecule symmetrical). ie each has 2/3rd of a negative charge and a partial double bond.
The Lewis structure there is bad because negative charge is very unlikely to reside primarily on the carbon because carbon is electropositive compared to oxygen!~
Can't see the drawing but remember that CO2 is linear with double bonds between the C and oxygens
obviously CO2 is symmetrical linear so has no overall dipole moment. If the drawing doesn't show that it's a bad description.
Again can't really answer without seeing. It should be linear, symmetrical, two double bonds to satisfy C's octet and the oxygens each have two lone pairs. No formal charges.
Se can expand its octet because its lower down in the periodic table. The compound is neutral so the 'real' structure won't give a formal charge to any atom. So I would say the best structure is the third one in the link above.
don't really understand the question. chemistry's not my major
as in structure 3 in the link above so it's bent
probably less than 120 because lone pair repels more than a bond would
It's like H2O because Se is in group 16 like oxygen.
How many lone pairs are then on H2Se (or H2O)? More or less than on SeO2? if more then it's going to repel more so the angle will be smaller.
They're not linear and symmetrical so yes they do.
My answers aren't perfect but I hope they help a bit