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why does chlorine have lower boiling point than bromine

My answer was "Chlorine has a smaller atomic radius which means it has weaker van der waals forces which is why it has a lower boiling point than bromine"

However according to the mark scheme i will only get 1/2 marks for saying it has a smaller atomic radius. The other mark is for saying that "The forces between chlorine / Cl2 molecules are weaker (than the forces between bromine / Br2 molecules)"

The mark scheme also says QoL in M2 for clear reference to the difference in size of the force between molecules.
Reference to Van der Waals forces alone is not
enough.

So why is mentioning van der waals wrong then since i'm just being more specific about what type of intermolecular force it is????

Here is the link to the question and mark scheme if you need it (Question 1d)
http://www.phoenix-tutoring.co.uk/data/documents/A2-AQA-Chemistry-2420-June-2015-CHEM2-Q01.pdf

Helppp plssss
anyoneeeeeeeeee
Marking scheme required mention of the fact that the forces are intermolecular. Just add a reference to the fact that the vdw forces are between Cl2 molecules.
Original post by golgiapparatus31
Marking scheme required mention of the fact that the forces are intermolecular. Just add a reference to the fact that the vdw forces are between Cl2 molecules.

so if i put intermolecular forces (vdW) between molecules are weaker , that would be okay?

Or would just van der waals between cl2 molecules be okay as well?
Original post by limitlesspuffy
so if i put intermolecular forces (vdW) between molecules are weaker , that would be okay?

Or would just van der waals between cl2 molecules be okay as well?

Yup, both things you said would earn you the 2nd mark. :smile:

The first mark is for a reference to size or number of electrons. (to explain why the IMFs are weaker)
Cl2 and Br2 only have weak van der waals intermolecular forces of attraction.
Br2 has a larger atomic radius, further down group 7, so Br2 is a bigger molecule than Cl2
Therefore, Br2 has greater v.d.w intermolec forces
Therefore more energy is required to overcome them, hence Br2 has a higher mp

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