Atoms of unusual size. In photosynthesis in plants, light is absorbed in light-harvesting complexes consisting of protein and pigment molecules. The energy absorbed is then transported to a specialized complex called the reaction center. Scientists believe that quantum mechanical effects may play an important role in this energy transfer. In a recent experiment, researchers used rubidium atoms cooled to a very low temperature to study a similar energy-transfer process in the laboratory. Laser light was used to excite an electron in each atom to a very highly excited state (large $n$ ). The excited electron behaves very much like the single electron in a hydrogen atom, with an effective $Z=1,$ but because $n$ is so large, the excited electron is quite far from the atomic nucleus, with an orbital radius of approximately $1 \mu \mathrm{m},$ and is weakly bound. Using these so-called Rydberg atoms, the researchers were able to study how energy is transported from one atom to the next in a process that may be a helpful model in understanding energy transport in photosynthesis.
The size of a highly excited atom can be taken to be the diameter of the orbit of the excited electron. If the researchers want to do the experiment with the rubidium atoms in a gas, with atoms separated by 10 times their size, the density of the atoms should be about
A. $10^{5}$ atoms $/ \mathrm{cm}^{3}$.
B. $10^{8}$ atoms $/ \mathrm{cm}^{3}$.
C. $10^{11}$ atoms $/ \mathrm{cm}^{3}$.
D. $10^{21}$ atoms $/ \mathrm{cm}^{3}$.