Solution: The diameter of a rubidium atom is 4.95 Å. We will consider two different ways of placing the atoms on a surface. In arrangement (a), all the atoms are lined up with one another. Arrangement (b) is called a close-packed arrangement because the atoms sit in the "depressions" formed by the previous row of atoms: How many Rb atoms could be placed on a square surface that is 5.5 cm on a side, using arrangement (b)?