The answer to whether copper is heavier than zinc depends on how “heavier” is defined. If comparing two equal volumes of the metals, copper is substantially heavier than zinc. However, when comparing a single atom of each element, the relationship reverses, and zinc is slightly heavier. This difference in mass across the atomic and bulk scales results from each element’s unique composition and physical structure.
The Atomic Weight Comparison
When comparing individual atoms, the measurement used is atomic weight, expressed in atomic mass units (amu). The average atomic weight reflects the weighted average of all naturally occurring isotopes. A single zinc atom has an average atomic weight of approximately 65.38 amu, meaning it contains more mass than a copper atom. Copper’s average atomic weight is about 63.546 amu. Therefore, based purely on the mass of a single atom, zinc is heavier than copper.
Understanding Bulk Density
In common usage, “heavier” refers to density, which is the amount of mass contained within a specific volume. Density is measured in units like grams per cubic centimeter (g/cm³). When comparing equal-sized pieces of each metal, copper proves to be significantly heavier than zinc.
Pure copper has a density of about 8.96 g/cm³, while pure zinc’s density is approximately 7.13 g/cm³. This means a copper object will weigh more than a zinc object of the exact same size. This difference confirms that, in a physical sense, copper is the heavier material.
How Atomic Structure Determines Mass
The reason copper is denser despite having a slightly lighter atom relates directly to the arrangement of its atoms. Copper atoms form a face-centered cubic (FCC) structure, which is a highly efficient way to stack spheres, filling approximately 74% of the available space. This close-packed arrangement allows a greater number of copper atoms to fit into a given volume.
In contrast, zinc forms a hexagonal close-packed (HCP) structure. Although HCP is also efficient, zinc atoms are slightly larger than copper atoms. The packing geometry results in a less compact overall arrangement. This less-dense packing of the slightly heavier zinc atoms ultimately makes a bulk sample of copper heavier per unit volume. The way the atoms stack together overcomes the small difference in the mass of individual atoms.