How come lead is denser than copper though more copper atoms can fit into a same size cube than lead atoms can?
Bob Pursley answered this question for another student. Here is a link. Ignore my answer (except the pictures may help). Mr. Pursley's answer is better than mine.
http://www.jiskha.com/display.cgi?id=1435608279
The density of a material is determined by its mass per unit volume. It is not solely based on how many atoms can fit into a given volume.
To understand why lead is denser than copper even though more copper atoms can fit into the same size cube than lead atoms, we need to consider the atomic mass and atomic size of each element.
Copper (Cu) has an atomic mass of around 63.55 atomic mass units (AMU), while lead (Pb) has an atomic mass of approximately 207.2 AMU. This means that each lead atom is significantly heavier than each copper atom.
Although more copper atoms can fit into a given volume, the individual atoms are lighter compared to lead atoms. This difference in atomic mass outweighs the difference in the number of atoms, resulting in lead being denser than copper.
In terms of atomic size, copper atoms have a smaller atomic radius compared to lead atoms. Therefore, a smaller volume is occupied by each individual copper atom, allowing more atoms to fit within the same space. On the other hand, lead atoms occupy a larger volume per atom, reducing the number of atoms that can fit into a given size cube.
To summarize, the higher atomic mass of lead compared to copper contributes significantly to the density difference, even though more copper atoms may fit into the same size cube due to their smaller atomic size.