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Question Video: Determining the Property That Gives Molecules the Ability to Form Hydrogen Bonds Chemistry

Molecules of HF, H₂O, and NH₃ are all able to form hydrogen bonds. What property of a molecule gives it the ability to form a hydrogen bond?

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Video Transcript

Molecules of HF, H2O, and NH3 are all able to form hydrogen bonds. What property of a molecule gives it the ability to form a hydrogen bond?

A hydrogen bond is a dipole–dipole interaction between a covalently bonded hydrogen atom and a lone pair of electrons on an adjacent molecule. Generally, this lone pair is from an electronegative element. Because atoms like fluorine, oxygen, and nitrogen are highly electronegative, when covalently bonded to hydrogen, a dipole is formed. The electronegative atoms tend to pull bonded pairs of electrons towards themselves, causing a partial separation of positive and negative charge, which forms the dipole.

The opposite partial charges on adjacent molecules are attracted to each other. The molecule with the partially positive hydrogen attracted to the adjacent molecule is the hydrogen bond donor. The molecule with a lone pair on an electronegative atom attracting an adjacent hydrogen atom is called the hydrogen bond acceptor. This attraction is known as the hydrogen bond. Hydrogen fluoride, water, and ammonia all form hydrogen bonds.

A hydrogen bond tends to be stronger if there is more pronounced charge separation in the molecules. For example, in hydrogen fluoride, the separation of charge is very high. Molecules with a hydrogen atom covalently bonded to fluorine tend to generate particularly intense hydrogen bonds with adjacent molecules. Molecules like these tend to form strong dipoles. This is due to the very high electronegativity value from the Pauling scale of fluorine, especially compared to that of hydrogen. Molecules of hydrogen fluoride contain covalent bonds with large differences in electronegativities and very strong dipoles.

Water molecules also engage in strong hydrogen bonds, with hydrogen and oxygen also having a large difference in electronegativity.

Ammonia molecules exhibit similar abilities to hydrogen bond.

These three molecules are all able to form hydrogen bonds from the difference in electronegativity of the atoms within them. Therefore, the property of a molecule that gives it the ability to form a hydrogen bond is a large difference in electronegativity between hydrogen and the atom it is bonded to.

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