Mercury (Hg) is denser than lead (Pb), a fact that may seem counterintuitive since mercury is a liquid and lead is a familiar, heavy solid. This difference is rooted in the physical property of density, which measures how much mass is packed into a given volume. Both mercury and lead are heavy metals, but their atomic structures cause them to behave differently when their densities are compared.
Understanding Density and Atomic Mass
Density is defined as the mass of a substance contained within a specific unit of volume, commonly expressed in grams per cubic centimeter (\(\text{g/cm}^3\)). This measurement quantifies how tightly atoms are clustered together; a higher density indicates a more efficient or compact atomic arrangement. While density is related to the mass of individual atoms, it is not solely determined by atomic mass.
Atomic mass refers to the weight of a single atom, reflecting the total number of protons and neutrons in its nucleus. Although atomic mass generally increases down the periodic table, density is also heavily influenced by crystal structure and the efficiency of atomic packing.
Comparative Density of Mercury and Lead
Mercury is significantly denser than lead, despite being a liquid at standard room temperature. The density of liquid mercury at \(20^\circ\text{C}\) is approximately \(13.546 \text{ g/cm}^3\). In contrast, the density of solid lead at the same temperature is about \(11.34 \text{ g/cm}^3\).
This means a volume of mercury weighs about \(20\%\) more than the same volume of lead. This difference is easily demonstrated because a solid piece of lead will float on liquid mercury. The lead floats because it is less dense than the mercury, displacing a volume of mercury with a greater mass than its own.
The high density of mercury is notable because liquids are generally less dense than their solid counterparts. For example, solid mercury, which forms below \(-39^\circ\text{C}\), has an even higher density of about \(14.18 \text{ g/cm}^3\). This shows that liquid mercury is already much denser than solid lead due to powerful underlying factors affecting its atomic structure.
The Atomic Basis for Mercury’s High Density
The reason mercury is denser than lead, despite having a lower atomic number (80 for mercury, 82 for lead) and a slightly lower atomic mass, lies in quantum mechanical effects. Mercury sits in a part of the periodic table where two distinct phenomena, the Lanthanide Contraction and Relativistic Effects, dramatically influence atomic size and packing efficiency.
The Lanthanide Contraction is caused by the poor shielding of the nucleus’s positive charge by the electrons filling the \(4f\) orbital. As the nuclear charge increases across the lanthanide series, the \(4f\) electrons do not effectively screen the outer electrons from the nucleus. This results in the outer orbitals, including the \(6s\) orbital in mercury, being pulled inward more than expected, leading to a smaller atomic radius.
This effect is compounded by Relativistic Effects, which become pronounced in heavy elements like mercury where inner-shell electrons move at a significant fraction of the speed of light. The increased speed causes these electrons to behave as if they have greater mass, which further contracts their orbitals. This contraction indirectly pulls the outer \(6s\) valence electrons closer to the nucleus, making the mercury atom smaller and its atoms more compact.
The combined effect of a smaller atomic radius and efficient packing results in mercury atoms packing much more efficiently than lead atoms. Even though lead has a greater atomic mass, mercury’s highly contracted atomic volume gives it a greater mass per unit of space.