Question

Both solid Kr and solid Cu consist of individual atoms. Why do their physical properties differ so much?

Short answer

The physical properties of solid Kr and solid Cu are different because Kr is a noble gas with complete octet configuration which renders weak London dispersion forces.

Step-by-step solution

Intermolecular force of attraction in solid Kr

Krypton (Kr) is a noble gas with complete octet, thus there are no ionic forces involved in the solid Kr and only weak intermolecular London forcesof attraction are present.

Difference in the physical properties

Solid Kr shows different physical properties than solid Cu, because in solid Kr only weak dispersion forces are present, whereas the solid Cu is not a noble metal and has two valance electrons. Thus, due to the difference in the intermolecular forces between the atoms, there is a difference in the physical properties.