# Homework Help: Harmonic vibrational frequency

1. May 10, 2010

### Ajs51

1. A molecule of 39^K 35^Cl consists of two ions,39^K+ and 35^Cl−,which are attracted to each other by electrostatic forces. The short-range repulsive force results in an interatomic distance of 2.7 ˚A at equilibrium. Considering only the electrostatic attraction, derive an expression for the force constant using Hooke’s law.Hence, determine the harmonic vibrational frequency of the 39^K 35^Cl molecule in wavenumbers.

3. I just cant seem to do this any help would be appreciated

2. May 11, 2010

There is one positive charge on the potassium and one negative charge on the chlorine. The attractive force is from these two opposite charges. It sounds to me like you are supposed to treat the repulsive force as being spring-like. You also know the distance between the atoms when the forces cancel each other out exactly, so that should give you all of the information that you need. Hopefully that points you in the right direction.

3. May 11, 2010

### Ajs51

Oh that makes sense..would it make sense to set the Coulomb force, equal to the hooke law...solve to find k...then use k to find w (angular frequency) ?

4. May 11, 2010

### Ajs51

Also thanks for the help!