Compute the volume of the carbon grain using the formula
V = (4/3)πr3
r = d/2
d = diameter
Look up the density of solid carbon, and multiply the volume of the grain by the density of carbon. That gives you the mass of the grain.
Look up the mass of a single carbon atom in a chemistry book. Divide the mass of the grain by the mass of a single carbon atom, and that gives you the number of carbon atoms in the grain.