Hydrogen bonding occurs between a hydrogen atom bonded to a highly electronegative element (such as fluorine, oxygen, or nitrogen) and a lone pair of electrons on another electronegative atom in a different molecule. The presence of hydrogen bonding depends on the electronegativity of the atoms and the availability of a lone pair of electrons.
In the case of HF (hydrogen fluoride) and C2H4 (ethylene):
- HF (Hydrogen Fluoride): In HF, the hydrogen atom is bonded to fluorine. Fluorine is highly electronegative, and it has a lone pair of electrons. The electronegativity difference between hydrogen and fluorine is significant, creating a strong dipole in the HF molecule. This results in hydrogen bonding interactions between HF molecules, where the hydrogen atom of one HF molecule is attracted to the lone pair of electrons on the fluorine atom of another HF molecule.
- C2H4 (Ethylene): In C2H4, which is a hydrocarbon, there are carbon-carbon double bonds and hydrogen atoms bonded to carbon. While carbon is not as electronegative as fluorine, oxygen, or nitrogen, it doesn't have a lone pair of electrons readily available for hydrogen bonding. The hydrogen atoms in C2H4 are not bonded to an electronegative atom with available lone pairs, so hydrogen bonding is not possible between C2H4 molecules.
In summary, the presence of hydrogen bonding in HF is due to the electronegativity difference between hydrogen and fluorine, as well as the presence of a lone pair of electrons on fluorine. On the other hand, hydrogen bonding is not possible in C2H4 because the hydrogen atoms are bonded to carbon, which does not have available lone pairs for hydrogen bonding interactions.