It exists, but due to the small masses involved is a distinctly second order effect compared to EM or the two nuclear forces. As such it can safely be neglected when examining the interactions of atoms with each other. At atomic scales, a full-up quantum theory of gravity will be necessary. However (as some ATM types who will remain nameless will gleefully tell you) there is still no fully satisfactory quantum theory of gravity. Even when one is developed it won't be necessary to invoke it on atomic scales.
"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind." - William Thompson, 1st Baron Lord Kelvin
"If it was so, it might be, and if it were so, it would be, but as it isn't, it ain't. That's logic!" - Tweedledee
This isn't right. This isn't even wrong. - Wolfgang Pauli