The diameter of a rubidium atom is 4.95A . 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 atoms could be placed on a square surface that is 4.5 on a side, using arrangement (b)?