3 A metal’s signature

In this section you are going to investigate how metals can be identified by the colour they impart to a flame. Electrons are found in shells around the nucleus. These shells are numbered 1, 2, 3 (and so on), moving outward from the nucleus (Figure 7). The number of the shell is known as the principal quantum number, and is given by the symbol n.

An electron’s energy depends on the shell it is in; in general as n increases, the energy of the electron also increases. The energy level occupied by electrons depend on the amount of energy in the system. At high temperatures (such as under a flame), electrons in the metal atoms will absorb heat energy and be promoted to higher energy levels.

Figure 7  (a) An electron jump from shell n = 2 to n = 3 is shown by the red arrow. (b) The electron jumps to a higher electron shell further away from the nucleus. Note that energy levels are not evenly spaced and they become closer together as they move away from the nucleus.

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This figure represents an electron jump from shell n = 2 to n = 3. The figure is in two parts. Figure 7(a) is a schematic diagram representing an atom. It shows a nucleus (a circle) at the centre, with three concentric rings that represent its shells labelled (from inside to out) n = 1, n = 2 and n = 3. An electron is shown in the n = 2 shell. The diagram illustrates (using an arrow) than this electron jumps from n = 2 to n = 3. In (b) there are three horizontal lines labelled (from bottom to top) n = 1, n = 2 and n = 3. An arrow to the side of the lines indicates that there is increasing energy from n = 1 to n = 3. The gap between n = 1 and n = 2 is around two and a half times bigger than that between n = 2 and n = 3, indicating more energy is needed to move between n = 1 to n = 2 than from n = 2 to n = 3. The electron jump from part (a) is shown as an arrow between the lines, pointing from n = 2 to n = 3.

Figure 7  (a) An electron jump from shell n = 2 to n = 3 is shown by the red arrow. (b) The electron jumps to a higher electron shell ...

The colour of the flame arises when these excited electrons return to lower energy levels emitting energy as light of a characteristic frequency. This gives a characteristic colour to the flame when a metal is heated in it (Figure 8). If you have ever let a pan containing salt in water boil over on a gas stove you may have noticed that the flame goes yellow; this is caused by the electrons in the sodium atom.

Figure 8  Flame test for sodium.

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A photograph of a Bunsen burner blue flame. A piece of white metal on the end of a wire extending from a glass rod is being heated in the flame to show the flame test for sodium. The blue flame is turning yellow with an orange glow around the piece of metal.

Figure 8  Flame test for sodium.

Visible light can be split by a prism into an uninterrupted band of colours, known as a continuous spectrum. However, the spectrum produced by the excited electrons of a particular element falling to lower energy levels consists of discrete coloured lines in a dark background. In Figure 9 you can see the emission spectrum of sodium when a beam of light from a sodium lamp is dispersed by a prism; the two intense yellow lines emitted by sodium atoms, are the main reason for the flame colour shown above.

Figure 9  Emission spectrum of sodium in the visible region.

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Figure 9 shows a spectrum. It is represented by a thin black rectangle in landscape format with five vertical coloured lines positioned along its length. From left to right the lines are: blue, green, two closely arranged yellow and red. The blue line is about a third along the rectangle. The green line is just over halfway along. The two yellow lines are about three quarters along. The red line is between the yellow ones and the end of the rectangle, but closer to the yellow lines.

Figure 9  Emission spectrum of sodium in the visible region.

Each element produces a unique set of spectral lines, and in the next section you will use the emission spectra of elements to identify some metals.