A black hole the size of an atom would be incredibly powerful, but its strength would depend on its mass. If we assume that the black hole has the mass of a typical atom, which is on the order of 10^-25 kilograms, it would have an extremely small Schwarzschild radius, which is the radius of the event horizon of a non-rotating black hole.
The Schwarzschild radius of a black hole is given by the formula:
r = 2GM/c^2,
where G is the gravitational constant, M is the mass of the black hole, and c is the speed of light. Plugging in the mass of an atom-sized black hole, we get:
r = 2(6.67430 × 10^-11 m^3 kg^-1 s^-2)(10^-25 kg) / (3 × 10^8 m/s)^2,
r ≈ 1.484 × 10^-57 meters.
This is an incredibly tiny distance, far smaller than the Planck length, which is about 1.616 × 10^-35 meters and is considered the smallest meaningful scale in physics.
Because of its extremely small size, the gravitational effects of a black hole the size of an atom would be negligible. It would have an exceedingly weak gravitational field, and its influence on its surroundings would be minimal. However, at such small scales, quantum mechanical effects become dominant, and our current understanding of physics, which is based on general relativity, breaks down. The behavior of black holes at such tiny sizes is not well understood and would require a theory of quantum gravity to accurately describe.