0.4 Quantum energy levels in atoms  (Page 7/9)

The shell and subshell organization of electron energies can also be observed by measuring the"electron affinity" of the atoms. Electron affinity is the energy released when an electron is added to an atom:

If there is a strong attraction between the atom A and the added electron, then a large amount of energy isreleased during this reaction, and the electron affinity is a large positive number. (As a note, this convention is the opposite of theone usually applied for energy changes in reactions: exothermic reactions, which give off energy, conventionally have negativeenergy changes.)

The electron affinities of the halogens are large positive values: the electron affinities of F, Cl, and Br are328.0 kJ/mol, 348.8 kJ/mol, and 324.6 kJ/mol. Thus, the attached electrons are strongly attracted to the nucleus in each of theseatoms. This is because there is room in the current subshell to add an additional electron, since each atom has 5 p electrons, and thecore charge felt by the electron in that subshell is large.

By contrast, the electron affinities of the inert gases are negative : the addition of an electron to an inert gas atom actually requires the input of energy, in effect, to force the electron into place. This is because the added electron cannot fitin the current subshell and must be added to a new shell, farther from the nucleus. As such, the core charge felt by the addedelectron is very close to zero.

Similarly, the electron affinities of the elements Be, Mg, and Ca are all negative. This is again because thes subshell in these atoms already has two electrons, so the added electron must go into a higher energy subshell with a much smallercore charge.

Electron waves, the uncertainty principle, and electron energies

We now have a fairly detailed description of the energies of the electrons in atoms. What we do not have is amodel which tells us what factors determine the energy of an electron in a shell or subshell. Nor do we have a model to explainwhy these energies are similar but different for electrons in different subshells.

A complete answer to these questions requires a development of the quantum theory of electron motion in atoms.Because the postulates of this quantum theory cannot be readily developed from experimental observations, we will concern ourselveswith a few important conclusions only.

The first important conclusion is that the motion of an electron in an atom is described by a wave function.Interpretation of the wave motion of electrons is a very complicated proposition, and we will only deal at present with asingle important consequence, namely the uncertainty principle . A characteristic of wave motion is that, unlike a particle, the wave does not have a definiteposition at a single point in space. By contrast, the location of a particle is precise. Therefore, since an electron travels as awave, we must conclude that we cannot determine the precise location of the electron in an atom. This is, for our purposes, theuncertainty principle of quantum mechanics. We can make measurements of the location of the electron, but we find that each measurement results in a differentvalue. We are then forced to accept that we cannot determine the precise location. We are allowed, however, to determine a probability distribution for where the electron is observed.