Chemistry, Third edition

3 · INSIDE THE ATOM

Evidence for the existence of energy

levels in atoms

You may have wondered what experimental evidence is available to support the

idea that atoms contain electrons which lie in shells at definite energy levels. The

truth is that the quantum theory of atoms was not arrived at quickly or easily, and

a short account of such evidence will inevitably appear sketchy and incomplete.

That said, there are two pieces of evidence which are difficult to explain without

quantization, and we shall briefly review them as they introduce important ideas

which will be useful later. They are successive ionization energiesandemission

spectra.

Successive ionization energies

Evidence for the existence of energy levels is provided by studies of ionization

energies (ionization enthalpies) which may be measured electrically or spectro-

scopically. The first ionization energy(symbolizedI 1 orH°ion(1)) of an atom in the

gaseous state is the minimum energy needed to completely remove the most easily

removed electron from the attraction of the atomic nucleus. The outermost elec-

trons possess more energy than the electrons in inner shells, and as a consequence

are least tightly held by the nucleus and require less energy to completely break

away from the attraction of the nucleus. It is for this reason that if energy is contin-

ually added to an atom, the outer electrons are ejected first.

Taking the potassium atom as an example, the first ionization energyis the

energy needed for the change:

K(g)K(g)e

2.8.8.1 2.8.8

Thesecond ionization energy(symbolizedI 2 orH°ion(2)) is the energy required for

the change:

K(g)K^2 (g)e

2.8.8 2.8.7

The third and successive ionization energies are defined in a similar way.

Figure 3.11 shows how the ionization energies for potassium change as we

3.5

40

Ionization energy

Write down an equation

representing the fourth

ionization energy of

beryllium (Be). Is there a

fifth ionization energy?

Exercise 3H

Fig. 3.11Successive ionization energies (kJ mol–1) for the potassium atom (2.8.8.1).