The atomic mass of an element represents the average mass of its atoms. However, density is not solely determined by the atomic mass of an element. Density is a measure of mass per unit volume, and it takes into account both the atomic mass and the atomic arrangement of the material.
In the case of iron (Fe) and lead (Pb), while lead has a higher atomic mass than iron, the densities of the two elements are not solely determined by their atomic masses. The density of a material is influenced by factors such as atomic packing, crystal structure, and interatomic forces.
Iron has a density of approximately 7850 kg/m³, which is lower than what one might expect based solely on its atomic mass. This is because the crystal structure of iron (specifically, the body-centered cubic arrangement) allows for relatively efficient packing of its atoms, resulting in a lower density compared to what would be expected based on atomic mass alone.
Similarly, lead has a density of approximately 11340 kg/m³, which is higher than what one might expect based solely on its atomic mass. The high density of lead is attributed to its close-packed crystal structure (face-centered cubic arrangement), which allows for a more compact arrangement of atoms.
Therefore, while the atomic mass of an element provides a rough estimate of its density, the actual density of a material is influenced by various factors beyond just atomic mass.