Hydrogen Bonding and Water Properties Notes

Hydrogen bonds are not covalent bonds and do not involve sharing or transferring electrons between atoms in the same way as covalent bonds. They are relatively weak interactions typically due to opposite charges attracting.
In water, hydrogen bonds form between a hydrogen atom covalently bonded to one oxygen and a nearby electronegative atom (often another oxygen) with lone pairs. Represented visually as a dotted line: ~~~ dot-dot-dot ~~~ (not a solid covalent bond).
Example discussed: water–ammonia interaction illustrating a single hydrogen bond where a hydrogen from water is attracted to the slightly negative region on ammonia.
A molecule of water can participate in multiple hydrogen bonds, depending on geometry and angles between molecules.
Hydrogen bonds are transient: they form and break rapidly, especially in liquid water. A humorous analogy used: “they last as long as a high school relationship.”
Significance: hydrogen bonding underpins many properties of water and its interactions with other molecules and surfaces.
Maximum capacity for hydrogen bonding by a single water molecule: 4 (two hydrogens as donors, two lone pairs on the oxygen as acceptors).
The only time water forms four hydrogen bonds is in ice, where the structure is tetrahedral; in liquid water, the typical instantaneous number of hydrogen bonds per molecule is around two, with continual breaking and reformation of bonds.

Water is a polar covalent molecule: oxygen is more electronegative than hydrogen, leading to partial negative charge on the oxygen and partial positive charges on the hydrogens.
Visual cue used in teaching: the oxygen atom has two small “Mickey Mouse ears” representing the two hydrogen atoms; the lone pair electrons on oxygen contribute to its partial negative charge.
The partial charges are denoted as δ+ on the hydrogens and δ− on the oxygen (in common pedagogy, hydrogens are slightly positive; oxygen is slightly negative).
How this leads to interactions: the positive hydrogen of one water molecule can be attracted to the negative region of another molecule or ion, forming hydrogen bonds.

Cohesion: water molecules hydrogen-bond to other water molecules (water to water). This gives water droplets a cohesive