Group 2: Alkaline earth metals
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Why does the magnesium ion not have a flame colour?
Jenkin replies
Read carefully the the section of the Nuffield Chemistry Students' Book headed 'Interpretation of the emission spectra of elements (pages 55 - 56). You will gather from this that when compounds of various metals are heated in a flame they vaporise. Some of the energy they are given is absorbed by the metal ion's electrons which are 'promoted' to higher energy levels.
We believe that electrons can only have certain specific energies (you might think of them in the same way as bottles on shelves: the bottles can only have a potential energy corresponding to the height of a particular shelf). When an electron falls back to a lower energy level, a quantity of energy corresponding to the difference between the higher and the lower energy level is given out. There is a mathematical relationship between the quantity of energy and its frequency:
Energy = a constant (called Planck's constant) x frequency
If you now look at the 'Background' diagram on p 57 of the Nuffield Chemistry book you will see that the frequencies of various kinds of energy, emitted as radiation, cover a very wide range of values. Visible radiation falls within a very narrow part of this range, which nevertheless includes frequencies emitted by several elements, such as K, Na, Li, Ba, Ca and Sr. It doesn't include magnesium because the frequencies of radiation emitted by its electrons fall outside the visible region, in either the infrared or ultraviolet regions.
Planck's constant is named after Max Planck, a German physicist, who was the first to suggest that energy may be quantised, i.e. only have certain specific values. Planck used the idea to explain the frequencies of energy emitted by hot objects (so-called black body radiation); it was Bohr who applied the idea to electronic energies.
Table 3.1 on pages 34 to 37 of the Nuffield Book of Data shows the electromagnetic spectrum (energies and frequencies of radiation) in much more detail.
Risk assessment
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updated: 22 November 2003
