Chemistry - Shapes of molecules

I came across an exam question which asked about the intermolecular forces within the molecules AsH3. The answer said there were van deer waals forces within it but not permanent dipole dipole attraction. I’m sort of confused because I thought AsH3 would be a polar molecules because it has a negative lone pair and positive hydrogen atoms.

Reply 1

angelxpink

9

Hi! So dipole-dipole interactions are caused by attractions between polar molecules. In order for a molecule to be polar there must be a large difference in electronegativity causing one atom to pull the electron density towards itself e.g. in water there is a large difference in electronegativity between the oxygen and the hydrogen in the O-H bonds hence oxygen pulls the electron density towards itself giving the oxygen a delta - charge and hydrogen a delta + charge hence it forms dipole-dipole intermolecular forces with other H2O molecules. There is not a large enough difference in electronegativity between As and H in the As-H bond so no permanent dipole forms hence dipole-dipole interactions don’t occur between AsH3 molecule and so the only intermolecular forces are VdW forces. Hope that helps. Let me know if u need any more clarification. Xx

Reply 2

dxnixl

18

In order for a molecule to be polar, two elements must have a large difference in electronegativity (usually bigger than 0.5). As the difference between H and As is 0.1, the dipoles cancel out, and the molecule is considered non-polar.

(edited 4 years ago)

Reply 3

Watermelon11

OP

9

Original post by angelxpink

Hi! So dipole-dipole interactions are caused by attractions between polar molecules. In order for a molecule to be polar there must be a large difference in electronegativity causing one atom to pull the electron density towards itself e.g. in water there is a large difference in electronegativity between the oxygen and the hydrogen in the O-H bonds hence oxygen pulls the electron density towards itself giving the oxygen a delta - charge and hydrogen a delta + charge hence it forms dipole-dipole intermolecular forces with other H2O molecules. There is not a large enough difference in electronegativity between As and H in the As-H bond so no permanent dipole forms hence dipole-dipole interactions don’t occur between AsH3 molecule and so the only intermolecular forces are VdW forces. Hope that helps. Let me know if u need any more clarification. Xx