Reading a character table

Science · Chemistry · ages 19-20

Name ______________________ Date ____________

In a character table, what does each column record?
- A vibrational frequency
- A class of symmetry operations
- An atomic mass
In a character table, what does each row record?
- One irreducible symmetry type
- One symmetry operation
- One molecule
Carbon dioxide is linear with 3 atoms. Type its number of vibrational modes.
Answer: ______________
Where does the symmetric stretch of carbon dioxide appear?
- Infrared only
- Both infrared and Raman
- Raman only
Sulfur dioxide is bent with 3 atoms. Type its number of vibrational modes.
Answer: ______________
A vibration shows up in the infrared spectrum when it changes what?
- The molecular mass
- The dipole moment
- The bond order
Reducing a representation means splitting the full motion into its irreducible row parts.
Circle one: True False
A centrosymmetric molecule has a mode that is strong in the infrared. What will its Raman signal be?
- Silent, by mutual exclusion
- Equally strong, since all modes are both
- Shifted to a higher frequency

Answer key

For grown-ups. Fold this page away before handing over the rest.

A class of symmetry operations · Columns group the symmetry operations into classes.
One irreducible symmetry type · Rows list the irreducible parts of the symmetry, one per row.
4 · Linear molecules have 3N minus 5 modes, so 9 minus 5 is 4.
Raman only · It changes polarizability with no dipole change, so it is Raman only.
3 · Nonlinear molecules have 3N minus 6 modes, so 9 minus 6 is 3.
The dipole moment · Light is absorbed only when the vibration moves charge asymmetrically.
True · Reduction breaks the whole into symmetry labelled pieces, one per row.
Silent, by mutual exclusion · With a centre of symmetry, no mode can be active in both spectra.