What causes certain metal ions to produce a ‘colored’ flame?
The atoms of every element contain electrons and the number of electrons in an atom is the same for a given element. The electrons in an atom can only exist in certain “states” each of which has a given energy. An electron, to move to a higher state, needs to absorb energy. To move to a lower state it much release energy – usually as radiation. If you give it enough energy, an electron can escape entirely leaving an ion (a charged atom) behind). This movement between energy states is called a transition. Each transition generates a certain amount of energy released as a single photon (bundle of light). The amount of energy in the bundle determines its frequency and hence (if visible) colour. Some atoms have fewer energy states available and hence fewer transition are possible. This tends to mean they have distinctive colours – e.g. yellow Sodium. Atoms with many possible energy states produce a mix of colours and hence are less distinctively coloures. Because ions have already lost one
