For solid solutions with the same arrangement of atoms, the size of a unit cell depends on the size of the atoms in the unit cell. The distance between nuclei also helps determine how strongly electrons are held, which changes the band gap energy. Samples of semiconductor CH[subscript 3]NH[subscript3]Pb(I[subscript 1-"x"Br"x")[subscript 3] solid solutions were prepared; the unit cell size was measured by powder X-ray diffraction, and the band gap energy was measured by visible absorption spectroscopy. This use of a material with significant research and technological implications for solar cells illustrates the periodic properties relationship between composition, atomic size, and electron energy levels.