Why does pf3 have dipole dipole internolecular forces? I thought it had dispersion...

I can't draw the Lewis structure on this board but here isw a link that will show you.

https://www.google.com/search?q=lewis+structure+PF3&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a&channel=sb

The molecular structure is here.
https://www.google.com/search?q=molecular+structure+PF3&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a&channel=sb

You can see it is not symmetrical; therefore, it has a dipole moment.

Thanks

PF3 does indeed have dipole-dipole intermolecular forces, but it also experiences dispersion forces.

Dipole-dipole forces occur between molecules that have permanent dipoles. In PF3, the molecule is trigonal pyramidal with one lone pair on the central phosphorus atom. The fluorine atoms in PF3 are more electronegative than phosphorus, creating a dipole moment in the molecule. The positive end of one molecule attracts the negative end of another molecule, resulting in dipole-dipole forces.

On the other hand, dispersion forces (also called London dispersion forces) exist between all molecules, regardless of polarity. These forces arise from temporary fluctuations in electron distribution, causing temporary dipoles. These temporary dipoles induce neighboring molecules to form temporary dipoles as well, leading to attractive forces.

In PF3, while dipole-dipole forces are the primary intermolecular forces, dispersion forces are present as well. The dispersion forces in PF3 are relatively weak compared to the dipole-dipole interactions, but they still contribute to the overall intermolecular forces in the substance.

To understand why PF3 (phosphorus trifluoride) has dipole-dipole intermolecular forces, it's necessary to consider the molecular geometry and the nature of the bonds within the compound.

Phosphorus trifluoride (PF3) has a trigonal pyramidal shape, with the phosphorus atom at the center and three fluorine atoms surrounding it. The shape arises due to the presence of one lone pair of electrons on the phosphorus atom, which creates electron-electron repulsion and causes the fluorine atoms to be pushed away from the central atom.

In PF3, the phosphorus atom is more electronegative than the fluorine atoms, resulting in a polar bond. The electronegativity difference between phosphorus and fluorine creates a dipole moment, where the phosphorus atom becomes slightly more negative (δ-) and the fluorine atoms become slightly more positive (δ+). This unequal distribution of charges within the molecule gives rise to individual molecular dipoles.

Now, turning to intermolecular forces, dipole-dipole forces occur when the positive end of one molecule attracts the negative end of another molecule. In PF3, the positive end of one molecule is the fluorine atom, which is attracted to the negative end of another molecule, represented by the lone pair of electrons on the phosphorus atom. This attraction between the partially positive fluorine atoms and the partially negative phosphorus atoms results in dipole-dipole intermolecular forces.

Additionally, while it is true that dispersion forces (also known as London forces) are present in all molecules, including PF3, dipole-dipole forces tend to be stronger than dispersion forces. This is because dipole-dipole interactions involve the attractive forces between the positive and negative ends of polar molecules, whereas dispersion forces result from temporary fluctuations in electron distribution that create temporary dipoles. So, in the case of PF3, the dominant intermolecular forces are dipole-dipole interactions.

In summary, PF3 has dipole-dipole intermolecular forces due to its polar bonds and the resulting unequal distribution of charges within the molecule. These forces are stronger than the dispersion forces present in all molecules.