Number 98 (Story #3), October 13, 1992 by Phillip F. Schewe and Ben Stein HOW MANY ATOMS DOES IT TAKE TO MAKE METAL? Small clusters of atoms have neither the properties of single atoms nor those of the bulk material. Several years ago studies of mercury clusters showed that the transition to metallic behavior occurred in the size range between 20 and 70 atoms. Now two different research groups in Germany have independently studied the optical properties of mercury clusters in this transition range. Scientists at the Philipps University at Marburg (K. Rademann et al.) and at Freiburg University (Hellmut Haberland et al.) report that as the cluster size is increased, an important absorption line in the mercury spectrum---that corresponding to a transition between the 6s and 6p states in single atoms---falls off because as the cluster grows, the two states start to overlap; meanwhile another (broad) absorption peak arises, corresponding to the excitation of a collective ("plasmon") electronic state in the cluster as a whole. (26 October 1992 issue of Physical Review Letters.)