Zinc had a higher atomic mass than copper, but copper's density is greater. Can someone explain this to me?
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Density is mass/volume. As mass increases, density increases. But since copper is more dense than zinc, this means the volume of a given mass of copper is smaller than the volume of a given mass of zinc. So the spacing between copper atoms in copper metal is smaller than the spacing between zinc atoms in zinc metal. This leads to the conclusion that bonding between copper atoms in copper metal is stronger than bonding between zinc atoms in zinc metal.
To understand this, use the "electron sea" model of bonding in metals. Metals are normally very conductive, meaning that one or more valence electrons are rather loosely held. Metals are also crystalline solids with the metal atoms tightly packed, implying strong bonding between metal atoms. In the electron sea model, the metal atoms are viewed as positively charged ions held together in a negative "sea" of freely moving valence electrons. A more sophisticated picture of this has the valence electrons delocalized in orbitals that span many metal atoms. Electron delocalization contributes to the strength of bonding between metal atoms.
In metallic copper, the copper atoms are more tightly packed (more strongly bonded to one another) than zinc atoms in metallic zinc. This can be explained by looking at the electron configuration of both atoms. Cu has the electron configuration of [Ar] 4s1 3d10. There is a lone 4s electron that is rather easily removed, contributing to the strength of the metallic bond since this electron is readily delocalized across multiple metal atoms. Zn has the electron configuration [Ar] 4s2 3d10. All electron shells are filled, which is a particularly stable electron configuration. The valence electrons are more tightly held, weakening electron delocalization among atoms in the metallic crystal. This also explains why metallic copper is more electrically conductive than metallic zinc.
Note that the same argument works for the atoms below Cu and Zn in the periodic table. Ag metal is more dense and a better electrical conductor than Cd metal. Au metal is more dense and a better electrical conductor than Hg. Ag and Au have a lone unpaired valence electron, like Cu. Cd and Hg have filled valence shells like Zn.
To understand this, use the "electron sea" model of bonding in metals. Metals are normally very conductive, meaning that one or more valence electrons are rather loosely held. Metals are also crystalline solids with the metal atoms tightly packed, implying strong bonding between metal atoms. In the electron sea model, the metal atoms are viewed as positively charged ions held together in a negative "sea" of freely moving valence electrons. A more sophisticated picture of this has the valence electrons delocalized in orbitals that span many metal atoms. Electron delocalization contributes to the strength of bonding between metal atoms.
In metallic copper, the copper atoms are more tightly packed (more strongly bonded to one another) than zinc atoms in metallic zinc. This can be explained by looking at the electron configuration of both atoms. Cu has the electron configuration of [Ar] 4s1 3d10. There is a lone 4s electron that is rather easily removed, contributing to the strength of the metallic bond since this electron is readily delocalized across multiple metal atoms. Zn has the electron configuration [Ar] 4s2 3d10. All electron shells are filled, which is a particularly stable electron configuration. The valence electrons are more tightly held, weakening electron delocalization among atoms in the metallic crystal. This also explains why metallic copper is more electrically conductive than metallic zinc.
Note that the same argument works for the atoms below Cu and Zn in the periodic table. Ag metal is more dense and a better electrical conductor than Cd metal. Au metal is more dense and a better electrical conductor than Hg. Ag and Au have a lone unpaired valence electron, like Cu. Cd and Hg have filled valence shells like Zn.