Despite the large difference in electronegativity between #"Si"# and #"F"#, #"SiF"_4# has a boiling point lower than that of #"NH"_3# (#-86^@ "C"#, vs. #-33^@ "C"#). Why is that? Could it be due to the symmetry? Hydrogen-bonding?

Question

Note: the #"Si"-"F"# bond length is #"155.4 pm"#, and the #"N"-"H"# bond length is #"101.2 pm"#.

Naomi Aronson · Accepted Answer

There are two answers

1.) Because of the hydrogen bonds (precisely interactions)

2.) Because of the tetrahedral arrangement of Si-F

Though the large difference in electronegativity between
#"Si" and "F"# the #"SiF"_4# (and even the bond length is more) has a boiling point lower than #NH_3# precisely because of hydrogen bonding.

This is because that boiling is completely dependent on intermolecular forces and not on intramolecular forces.

#"SiF"_4# is nonpolar though #"Si"-"F"# is polar because the tetrahedral arrangement of four Si-F cancel out the dipoles rendering the #"SiF"_4# of zero dipole. This means that #"SiF"_4# is nonpolar and London Dispersion Forces are the force than act on these molecules

And in #"NH"_3# there are hydrogen bonds which are a special case of dipole-dipole forces.

Because that London Dispersion Forces are temporary because electron density is always changing all across the atom dipole-dipole forces are much stronger because they are permanent and always in alignment.

Despite the large difference in electronegativity between #"Si"# and #"F"#, #"SiF"_4# the electronegativity doesn't matter because of the tetrahedral of four Si-F make the the compound nonpolar.

And more stronger the intermolecular force more the boiling point

Kai Bowman · Answer

undefined The difference in boiling points between silicon tetrafluoride (SiF4) and ammonia (NH3) can be attributed to several factors. While the electronegativity difference between silicon (Si) and fluorine (F) is significant, other factors such as molecular weight and molecular geometry also play a role. In the case of SiF4, its larger molecular weight contributes to stronger van der Waals forces, which can increase its boiling point compared to NH3. Additionally, the molecular geometry of SiF4 is tetrahedral, leading to weaker intermolecular forces compared to the more polar and asymmetric NH3 molecule, which can also affect the boiling point. Hydrogen bonding, a significant factor in NH3, is not present in SiF4 due to the absence of hydrogen atoms. Therefore, while electronegativity differences are important, other molecular properties also influence the boiling points of compounds.