The ability of free atoms to absorb

A given population of free atoms exists at various electronic energy levels. The distribution of atoms in the energy levels is given by the Boltzmann distribution equation, i.e. 

The equilibrium between atoms in the high energy state and atoms in the low energy state depends on the energy required to excite the atoms and the temperature of the system. If the energy is great, the number of excited atoms is small and vice versa. If the temperature is high, the number of excited atoms is increased. 

At room temperature and at the temperatures obtained by most flame atomisers the number of atoms in the excited state is a very small fraction of those in the ground state and for absorption purposes can be ignored. We will discuss this relationship later. 

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