Micro black holes, which means small, unstable black holes like they will propably be created in the LHC have a minimum mass of 10^16 TeV (10^-8 kg). The mass of a proton is 1,67*10^-27 kg, so you need about 6*10^18 protons/nucleons. So you need way more that a single atom.

http://de.wikipedia.org/wiki/Micro_Black_Hole
http://de.wikipedia.org/wiki/Proton