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Question

The work function of an element is the energy required to remove an electron from the surface of the solid element. The work function for lithium is 279.7 kJ/mol (that is, it takes 279.7 kJ of energy to remove one mole of electrons from one mole of Li atoms on the surface of Li metal). What is the maximum wavelength of light that can remove an electron from an atom on the surface of lithium metal?

Answer 1

What you really need is the work function per Li atom. That can be calculated from 279.7 kJ / mole divided by the number of atoms in a mole (Avagadro's number).

Once you have that energy (and it will be quite small, probably around 2.9 electron-Volts) you can find the frequency of a photon with that amount of energy using the energy / frequency relationship

E = h f

where E is the energy of the photon, h is Planck's constant, and f is the frequency.

If you want wavelength instead of frequency, use the relationship

c = f l

where c is the speed of light, f the frequency, and l the wavelength.

If you need to convert between electron-Volts and Joules, know that there are 1.602 x 10 ^ -19 J per electron-Volt.