The total internal energy of an object is the sum of the average energy of each of the atoms that make up the object. Hence, at certain temperature , it is possible that some atoms are more energetic than others.(These atoms occupy a high energy level than others)

To find the ratio of number of atoms at a higher energy level E_{x} to the number of atoms at a lower energy level E_{o}:

$\frac{N_{x}}{N_{o}} = e^{ – \frac{E_{x} – E_{o}}{kT}}$, where

N_{x} is the number of atoms with energy E_{x}

N_{o} is the number of atoms with energy E_{o}

k is the Boltzmann constant: 1.381 x 10^{-23} J/K

T is the temperature in K