Density
Mass per unit volume at standard conditions — how tightly an element's atoms are packed together.
See density colored on the interactive table →Across a period (left → right)
Tends to rise through the transition metals, then fall toward the lighter nonmetals.
Down a group (top → bottom)
Generally rises down a group.
Why it happens
Density is a tug-of-war between atomic mass (which grows fairly steadily as you add protons and neutrons) and atomic volume (how much space each atom actually takes up, including how tightly the solid packs them). Osmium and iridium, both dense transition metals, combine a heavy nucleus with a compact metallic structure to reach the highest densities of any element. Down a group, added mass usually outpaces the modest increase in atomic size, so density tends to climb.
Elements that are gases at room temperature have very low densities in this dataset regardless of their position on the table — density here means the STP state, not a hypothetical solid form.