Different metals produce different coloured flames when burnt in oxygen due to the unique electronic structures and energy levels of each metal. When heated, the valence electrons within the metal atoms absorb energy and become excited. As these excited electrons return to their ground state, they release energy in the form of photons of light, producing a characteristic colour depending on the wavelength of the emitted light.

Here are the reasons why metals exhibit different flame colours:

1. Electronic Configuration: The electronic configurations of metals determine their excitation energies. Metals with loosely bound valence electrons (low ionization energies) tend to emit longer-wavelength, lower-energy photons, resulting in colours towards the red end of the spectrum. Metals with tightly bound valence electrons (high ionization energies) emit shorter-wavelength, higher-energy photons, producing colours towards the blue or violet end of the spectrum.

2. Atomic Structure and Bonding: The crystal structure, atomic size, and bonding properties of metals also influence the flame colour. The interactions between metal atoms and the surrounding oxygen molecules during combustion affect the energy levels and transitions of the excited electrons, leading to variations in colour.

3. Vibrational and Rotational Energy Levels: In addition to electronic transitions, the vibrations and rotations of molecules within the flame can contribute to the overall flame colour. Different metals produce flames with different temperatures, which influence the extent of vibrational and rotational excitations, resulting in additional spectral features and colour variations.

4. Partial Combustion: Some metals undergo incomplete combustion, where only partial oxidation occurs, leading to the formation of various chemical species in the flame. These chemical species can emit their own characteristic colours, contributing to the overall flame colour observed.

5. Impurities and Contaminants: The presence of impurities and contaminants in the metal or the combustion environment can also influence the flame colour. Trace elements or compounds within the metal can introduce additional emission lines or spectral bands, altering the perceived flame colour.

6. Temperature: As the temperature of the flame increases, the energy of the emitted photons increases, resulting in a shift of the flame colour towards the blue end of the spectrum. Higher temperatures excite electrons to higher energy levels, leading to the emission of shorter-wavelength, higher-energy light.

It's worth noting that the colours observed may not always be pure spectral colours but can appear as mixtures or combinations due to the presence of multiple emission lines and the overlapping of different colours. The exact flame colour produced by a particular metal may vary depending on experimental conditions, such as the fuel-to-oxygen ratio, temperature, and surrounding atmosphere.