Final answer:
Hydrogen bonds occur when hydrogen atoms in polar covalent bonds are left with a partial positive charge due to electronegative elements pulling electron density away, allowing hydrogen to bond with lone pairs on nearby electronegative atoms.
Step-by-step explanation:
Hydrogen bonds form in molecules where hydrogen is covalently bonded to highly electronegative elements, namely fluorine (F), oxygen (O), or nitrogen (N). These elements have a strong attraction for electrons, drawing the electron density away from the hydrogen atom within the bond, which in turn leaves hydrogen with a positive partial charge. Attraction then occurs between this positively charged hydrogen and a lone pair of electrons usually belonging to F, O, or N on a nearby molecule or within the same molecule, resulting in a hydrogen bond. This bonding is essential in water (H2O), where the hydrogen and oxygen atoms generate partial positive and negative charges, respectively, due to the difference in electronegativity, thus allowing for the formation of hydrogen bonds between water molecules. The unique properties of water, DNA structure, and other natural phenomena are greatly influenced by the presence and properties of hydrogen bonds.