Elements Listed by Density

Density is mass per unit volume. When you think about the density of pure elements, you would expect elements that are gases would be the least dense, progressing across and down the periodic table to form elements that are more dense. This is generally true, but if you compare the list of element densities with the periodic table, you'll see there are exceptions almost immediately. For example, beryllium, the fourth element, is the 19th element listed (8th non-gas). The packing of the electrons figures promininently into density. Elements that pack their electrons into neat, complete shells or can stack atoms close together are more dense than those with larger, incomplete electron shells.
Elements Listed by Density | Chemistry Charts & Tables