At ##20°C##, the density of copper is ##8.9## ##g####/####cm^3##. The density of platinum is ##21.4## ##g####/####cm^3##. What does this tell you about how the atoms are "packed" in each material?

Good question. Remember, that we compare a 3rd row transition metal ##(Pt)##, to a 1st row transition metal ##(Cu)##.

##"Molar mass (Cu)"## ##=## ##63.55## ##g*mol^-1##.

##"Molar mass (Pt)"## ##=## ##195.08## ##g*mol^-1##.

This threefold difference in molar mass, of course translates to . Given that the difference in densities is (very approx.) threefold as well, we would predict that the atomic packing in both metals is similar in terms of , ##"mass per unit volume"##. Does this make sense?