Today we went on a lab outing – yachting – and I was surrounded by great company – countless water molecules! The lab mates are cool too 🤓 We often take WATER for granted, but it’s SOOOOOO important – for basically everything. When scientists look for life on other planets they usually check for water because it has unique properties that make it crucial for life as we know it. Some of these (such as floating as a solid) may seem magical, but it can all be explained by chemistry! And some interatomic tug-of-war!
Molecules like water are made up of smaller things called atoms – in water there are 2 hydrogen (H) atoms and – oxygen (O) one. And atoms join together by sharing even smaller things called electrons. But they don’t always share fairly. Instead you can think of each interatomic interaction as a game of tug-of-war for pairs of e⁻. With the “victor” chosen based on electronegativity (ability to attract e⁻) rather than muscle
Sometimes, players are equally matched (such as hydrogen (H) & carbon (C)). They share pretty equally, giving you strong COVALENT bonds. These sort of bonds keep our macromolecules (e.g. sugars, fats, & proteins) together, These bonds are really strong because each atom has significant stakes in the game.
But in other tug-of-wars, one player’s much “stronger” – it yanks imaginary rope so hard the other atom lets go. This is the case with the really electronegative element, chlorine (Cl), which takes an e- from Na in table salt for an IONIC bond. While we call often call these “bonds” they’re not actually sharing electron housing – it’s actually just a really strong attraction,
That attraction comes because electrons are negatively-charged. And if there aren’t enough positively-charged protons to counterbalance them, the atom that gave up an e⁻ is now ➕ (CATIONIC) & the atom that took it’s now ➖ (ANIONIC). Opposite charges attract so you have “electrostatic interactions” that lead them to stay together to form solid crystal we call SALT!
Mostly things are somewhere in-between truly equal sharing and extremely unfair sharing, and this is case w/H₂O. O’s more electronegative (stronger electron-pulling power) than H BUT it’s not *so* much more that H will surrender e⁻ entirely. So they compromise. They partly share a pair of e⁻ but they’re “held” closer to O. We talk about being electrons being held, but in reality electrons are constantly whizzing around and if one atom in a bond “holds electrons closer” it means you’re more likely to find an electron closer to it. This sort of bond is called a POLAR COVALENT BOND
And since the electrons hang out near the O more than near the H, the H is partially➕ (δ⁺)(pronounced delta negative) & O partially ➖ (δ⁻). In situations like this where there’s a charge difference between different parts of molecules we call it a DIPOLE. And we call molecules with dipoles POLAR (hence the name polar covalent bond – it’s polar because the bond has “poles” and its covalent because they’re sharing electron housing situations.
These aren’t “full” charges, but they’re still charges that can attract & repel other charges. Including those partial charges in nearby water molecules – the δ⁺ H are attracted to δ⁻ O & vice versa. These aren’t covalent bonds but they do have another special name. They’re an example of a HYDROGEN BOND (H-bond).
H-bond is the name we give to dipole-dipole attractions where you have the partly positive thing being a H attached to and electronegative atom (like oxygen) and the partly negative thing being an electronegative atom w/lone pair of e⁻ (e.g. O or N).
Since they’re just partial charges and not full charges like in ionic bonds, they’re weaker than ionic bonds (which are weaker than covalent bonds) but H-bonds are some of the strongest dipole-dipole interactions. Individually they may be “weak” but together they’re VERY strong. They’re crucial for MUCH of biochemistry – & life! (e.g. H-bonds zip strands of DNA together)
If oppositely-polar parts of molecules like each other enough they’ll stick around. But molecules are fickle friends – what they really want is freedom so they like to explore & don’t like to be tied down – even if they like their partner. But they need energy to overcome the attraction.
In liquid water the H₂O molecules have too much energy for these HYDROGEN BONDS (H-bonds) between molecules to stick around long BUT at low temps they can’t move as fast. So they stick together in a crystal lattice we call ICE!
Unlike other molecules, liquid water’s less dense as a solid because in order to take full advantage of their H-bonding potential, H₂O molecules have to arrange themselves to line up dipoles and this requires spreading out more than typical solid packing. This leaves more room for air so ice is lighter than liquid water – so it floats!
And if you go the other direction from liquid water – add energy – the molecules are free to explore – the H-bonds are no match for the drive to explore – so the molecules break free from one another and escape into the “gas” form. And we call gassy water water vapor. And there’s actually a lot of it in the air around us!