Atoms are too small to be seen. We can only obtain information indirectly by stimulating the atoms with energy of various forms and detecting what emerges.
Evidence for the existence of energy levels.
Scientists deal with a microscopic world that can never be seen. So how do they "know" the structure of an atom?
It is rather like a birthday present that arrives well wrapped. The recipient may try to guess the contents of the package without actually opening it. Many people shake the box, listen to the movement of the contents, try to gauge the weight and how the contents move around in the box.
What they are doing is putting energy into the box and by sensing the response from the box they hypothesise (guess) what the contents are.
Its the same procedure with scientists and the microscopic world.
Energy is given to the particles (atoms, molecules or ions) and the response from the unseen world is detected. Hypotheses are formulated which are then tested by all available means. If the experimental evidence seems to fit the hypothesis then a theory is proposed which in time comes to be accepted as the "truth".
The three pieces of evidence are as follows:
- 1 Spectra
- 2 Successive ionisation energies
- 3 Ist ionisation energies of the different elements
Different types of spectra have already been covered in section 1.31. These may be summarised as:
- 1 continuous emission
- 2 continuous absorption
- 3 line emission
- 4 line absorption
Line emissions arise from energy being emitted by the atom as a result of electrons in high energy levels returning to their ground states. The electrons must emit the difference in energy and this is seen as light (electromagnetic radiation) of a specific wavelength. They first arrive at the higher level by absorbing energy in the form of either heat or electricity.
- An electron absorbs heat or electrical energy and is promoted to a higher level
- The electron returns to the original level and emits the difference as a specific electromagnetic radiation.
- The wavelength seen is related to the energy of the emission by Plancks equation E=hn
- E = energy of the emission
- h = Plancks constant (6.63 x 10-34)
- n = frequency of the radiation (the frequency is related to the wavelength by c = ln, c is the speed of light and l is the wavelength)
The ionisation energy of the elements can be determined by several means. These are beyond the scope of the Syllabus. Students must understand the definitions of 1st and successive ionisation energies and also the factors that affect them, specifically electrostatic forces.
The first ionisation energy
The first ionisation energy is defined as the energy required to remove one mole of electrons from one mole of gaseous atoms to provide one mole of gaseous single charged ions.
|Na(g) Na+(g) + 1e|
Subsequent ionisation energies are defined in a similar way only by removing electrons from already charged ions.
The second ionisation energy
|Na+(g) Na2+(g) + 1e|
Successive electrons can be stripped from an atom until there is only the nucleus left. If the energy required to achieve this for each ionisation is plotted on a graph (with a log scale) against the ionisation number, the 'jumps' in the required energy clearly show the main and sub energy levels.
In this example, it may be seen that removal of the first electron requires (relatively) less energy than removal of the next (eight) electrons - there is a distinct inflexion (change of direction) in the otherwise fairly linear graph. Consequently the element concerned must be in group I.
Many exam questions focus on the ability of a student to recognise this inflexion from purely numerical data and then ask for details of its group in the periodic table.
Example: In the following table identify the groups to which the elements X, Y and Z belong (all values in kJ mol-1).
It may be seen that the inflection (relatively bigger jump) for element X occurs between 1st and 2nd ionisation energies. It is therefore in group 1. Similarly the inflection for Y occurs between the 2nd and 3rd ionisation energies and so it is in group 2.
1st ionisation energies of successive elements
The first ionisation energy of an element is defined as the energy required to remove 1 mole of electrons from one mole of gaseous atoms under standard conditions.
M(g) M+(g) + 1e
A graph of first ionisation energy against atomic number shows how the first ionisation energy varies moving from element to element in the periodic table. The outermost electron is being removed in each case and so the amount of energy needed to remove it is a function of the force holding the electron in position around the atom.
This force is dependent on two main factors and is 'fine-tuned' by a third factor.
- 1 The charge on the nucleus
- 2 The distance of the outer electron from the nucleus
- 3 Inter-electron repulsions
As the charge on the nucleus increases so the energy required to remove the electron increases.
As the distance between the outermost electron and the nucleus increases so the energy required to remove it decreases.
The graph is probably best understood by referring to specific examples. Click on the element values on the graph below to find out more.
Spectral lines give evidence of electrons moving from one energy level to another within the atom.
Successive ionisations of an atom suggest that there are energy shells with large energy differences between them.
The 1st ionisation energies of the first 36 elements suggests that the energy shells are split up into sub-shells and that some of these sub-shells have further divisions (orbitals).