Calculate the relative rate of effusion of ²³⁵UF₆(g) to ²³⁸UF₆(g) using sections 1 and 6 of the data booklet.

Explain, based on molecular structure and bonding, why diffusion or centrifuging can be used for enrichment of UF₆ but not UO₂.

M_r(²³⁵UF₆) = 235 + (19.00 × 6)/349

OR

M_r(²³⁸UF₆) = 238 + (19.00 × 6)/352

«\(\frac{{{\text{rate of effusion of}}{{\text{ }}^{235}}{\text{U}}}}{{{\text{rate of effusion of}}{{\text{ }}^{238}}{\text{U}}}} = \sqrt {\frac{{352}}{{349}}}  = \)» 1.004

Award [2] for correct final answer.

Do not accept “1.00” OR “0.996”.

[2 marks]

UF₆: Structure: octahedral «solid»/square bipyramidal «solid»/«simple» molecular solid/simple molecule AND Bonding: covalent

UO₂: Structure: crystal/lattice/network «solid»/«resembles» fluorite AND Bonding: «partly» covalent

UF₆ sublimes/evaporates/boils at low temperature

Accept “UF₆: Structure: octahedral «solid»/square bipyramidal «solid»/«simple» molecular solid/simple molecule AND weak intermolecular/London/dispersion/van der Waals’/vdW forces”.

Accept “non-polar molecule” for “«simple» molecular solid”.

Accept “giant molecular” OR “macromolecular” for “network”.

Accept “ionic/electrostatic attractions «between ions»” for bonding in UO₂.

Award M2 for “UO₂: network covalent/covalent network/giant covalent” OR “UO₂: network ionic/giant ionic”.

For M1 and M2 award [1 max] for two correct structures OR two bonding types.

Accept any specified low temperature in the range 56–65 °C.