[rosco_pk]

Hi there, I hope someone can help. I have been asked to make a carefully annotated sketch of the Lennard-Jones potential for Xe. I have been supplied with values for σ (0.406 nm) and ε ( 3.16 x 10^-12 J). I have consulted my lecture material and also searched online but can find nothing relating to how to sketch the diagram using these figures. I have also been asked to calculate and mark on the diagram the equilibrium separation of Xe₂, but without the diagram I don't know how to go about doing this. I am not asking you to answer the question, some pointers in the right direction would be helpful though. Thanks.

[Yggdrasil]

Do you know the equation for the Lennard-Jones potential?  (If not, then consult your lecture material, textbook, or wikipedia[http://en.wikipedia.org/wiki/Lennard-Jones_potential])

The equilibrium separation occurs when the potential energy is minimized.  Do you know how to find the minimum of a curve (hint: think back to calculus)?

[rosco_pk]

Yeah I know the equation, I don't know what value to use/how to calculate r. I suppose that should really have been my question. I know it is something to do with repulsion. I missed the lecture you see and only have minimal powerpoint slides to work from!

Thanks with the pointer about minimum curve. I know how to do that! Cheers.

[Yggdrasil]

r is your independent variable.  When you plot the LJ potential (V), you will have V on the y-axis and r on the x-axis.  r represents the distance between the two Xe nuclei, so your graph will show how the interaction energy between the two Xe atoms changes as you bring the two atoms closer or move them apart.

[rosco_pk]

Excellent, that has helped a lot! I don't normally like to ask and try to figure it out myself but this had me stumped!