The atomic ratio is a measure of the ratio of atoms of one kind (i) to another kind (j).
A closely related concept is the atomic percent (or at.%), which gives the percentage of one kind of atom relative to the total number of atoms.
Another application is in radiochemistry, where this may refer to isotopic ratios or isotopic abundances.
Mathematically, the isotopic abundance is where Ni are the number of atoms of the isotope of interest and Ntot is the total number of atoms, while the atomic ratio is For example, the isotopic ratio of deuterium (D) to hydrogen (H) in heavy water is roughly D:H = 1:7000 (corresponding to an isotopic abundance of 0.00014%).
In laser physics however, the atomic ratio may refer to the doping ratio or the doping fraction.