The Shell Model

And Photoelectron Spectroscopy and the Structure of Atoms Ch 7 part 1.5

A little history

 Einstein earned the Noble Prize for his work on the Photoelectric effect.

 The photoelectric effect is when electrons are emitted from a solid liquid or gas when they absorb energy in the form of light.

 This has been determined experimentally by shining light on metal.

Photoelectric Effect By shining a light on metal, one may create a current

Photoelectric effect

  Red light will not cause the ejection of electrons, no matter what the intensity!

 Increasing the intensity of the light increased the number of photoelectrons, but not their maximum kinetic energy A weak violet light will eject only a few electrons, but their maximum kinetic energies are greater than those for intense light of longer wavelengths!

E photon = h v

Photoelectron Spectroscopy

 PES provides a useful means to extract information on atomic structure using spectroscopic data.

 By looking at the data chart one may determine the structure of the atom.

 How? The chart gives evidence for the shell model.

Photoelectron Spectroscopy (PES)

 Do all electrons in a given shell have the same energy?

 To answer this a technique using radiation on a neutral atom in the gas phase called Photoelectron Spectroscopy is used.

 The technique shines radiation (UV or X ray) on an atom to excite it to the point that it will eject an electron from its shell and form a cation.

Shell Model

  In the shell model, we see the ionization energy is decreased the farther the electron is from the nucleus.

 By the way of reasoning, it takes more energy to remove an electron from n=2 than n=3.

Photoelectron Spectroscopy (PES)

 The energy of the photon used is higher than the ionization energy.

 The excess energy is converted to the kinetic energy on the ejected electron.

 The energy required to remove the electron is equal to the difference between the energy absorbed by the atom (hv) and the kinetic energy of the ejected electron  IE = hv - KE

Photoelectron Spectroscopy (PES)

 This technique allows the researcher to remove one electron from any shell.

 Therefore, One may remove electrons from the inner core without moving the outer electrons.

The apparatus

Reading a PES spectra

 On the PES spectra, energy increases as one moves to the left. The height refers to the number of electrons.

Demostration

 http://www.chem.arizona.edu/chemt/Flas h/photoelectron.html