increase the number of electrons removed. (The log of the ionization energies has
been plotted in order to compress the ionization energies so as to make the y-axis a
reasonable size.)
Each point in Fig. 3.11 represents one electron. The obvious feature is that the
points are clumped together into four groups. This provides direct evidence that the
19 electrons in the potassium atom belong to four groups. These groups are the electron
shells.Moving from left to right in Fig. 3.11, the groups contain one, eight, eight
and two electrons, respectively. This is the reverse of the familiar 2.8.8.1 configura-
tion for potassium (reversebecause in ionization the outer electrons are removed
first). Although we have only plotted the ionization energies for potassium, similar
patterns are seen for atoms of the other elements.
Emission spectra of atoms
Evidence for the quantization of electrons in atoms also comes from emission
spectra. If a sample of atoms is heated, or exposed to light of a suitable wavelength,
the outer electrons of some atoms may gain energy, and the atom is then said to be
in an excited state. One way in which this energy may be lost is by emission, in
which the electron loses energy as light. (The yellow colour seen when common
salt is sprinkled into a flame is an example of such an emission.)
The frequency (, pronounced ‘nu’) of the emitted light depends upon the
amount of energy lost by the electron, E. The exact relationship between the two
quantities is described by the Planck equation:
Ehwherehis a universal constant known as the Planck constant.
If the emission of light from an element is studied using an instrument which
allows the measurement of the frequency of light (a spectrometer), it is noticed
that several frequencies of light are usually emitted: these frequencies make up the
emission spectrum of the element. Just as importantly, however, there are some
frequencies which are not emitted. This means that only some values of energy
changeEare allowed. The simplest way of explaining this is to assume that the energy
of the electron in the atom may only possess certain values. In other words, the energy
of electrons in atoms is quantized. (More details about light emission, and calcula-
tions using the Planck equation are given in Chapter 20.)
More advanced ideas about electronic
structure
a
Sublevels of energy
According to the Bohr picture of the atom, all electrons in the same shell possess
the same energy. More detailed experiments (also involving ionization energies and
spectra) show that with the exception of electrons in the first shell this is not the
3.6
MORE ADVANCED IDEAS ABOUT ELECTRONIC STRUCTURE 41aThese ideas are not essential for an understanding of Unit 4.