Atomic spectra and flame tests

    In the vapour state, the alkali metals exist as atoms or M₂ molecules. The strength of the M^_M covalent bond decreases down the group. Excitation of the outer ns¹ electron of the M atom occurs easily and emission spectra are readily observed. We have already described the use of the sodium D-line in the emission spectrum of atomic Na for specific rotation measurements. When the salt of an alkali metal is treated with concentrated HCl (giving a volatile metal chloride) and is heated strongly in the non-luminous Bunsen flame, a characteristic flame colour is observed (Li, crimson; Na, yellow; K, lilac; Rb, red-violet; Cs, blue) and this flame test is used in qualitative analysis to identify the M‏ ion. In quantitative analysis, use is made of the characteristic atomic spectrum in flame photometry or atomic absorption spectroscopy.