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What causes differing density?

Is one metal more dense than another because the atoms are larger (more mass per atom), or because the atoms are closer together? Or both?

Example: Aluminum
Density: 2.7 g/cm³
Atomic mass: 26.981539 u
Protons: 13
Neutrons: 14

Copper
Density: 8.96 g/cm³
Atomic mass: 63.546 u
Protons: 29
Neutrons: 35

So the atomic mass of copper is higher than aluminum because it has more protons and neutrons.

I have also read that the atomic spacing in copper is approximately 2.56 angstroms (Å), while the atomic spacing in aluminum is approximately 2.86 Å.

So it seems the atoms in solid copper are closer together than in aluminum.

Can you confirm this all sounds right and add any comments to help us understand why different metals have different densities?

Thanks!

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    $\begingroup$ Both.$\mathstrut$ $\endgroup$ Commented Mar 9 at 20:02

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Your explanation is right. I must add that most metals also contain differences in their chemical bonding, which affect their bond lengths. Being a metal does not exclude also having covalent bonds. Covalent bonds are shorter than purely metallic bonds.

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