A Bond Angle and a Habitable Planet

Hydrogen sulfide has a molecular weight of 34. Water has a molecular weight of 18. All else being equal, the heavier molecule should be harder to boil.

Hydrogen sulfide boils at minus 60 degrees. Water boils at 100. The gap is 160 degrees, and everything that makes the Earth habitable is in it.

The reason is that oxygen pulls shared electrons much harder than hydrogen does. In a water molecule the electrons spend more time near the oxygen, leaving it slightly negative and the two hydrogens slightly positive. The molecule is bent rather than straight (about 104.5 degrees) so the charges do not cancel, and the whole thing is a small permanent dipole.

Water molecules therefore attract each other, positive hydrogen to negative oxygen, and each molecule can make up to four such connections. These hydrogen bonds are perhaps a twentieth as strong as a covalent bond, and there are an enormous number of them, and pulling a water molecule away from its neighbours to become a gas is correspondingly hard.

Almost every peculiar property of water traces to this. Its high boiling point, which keeps oceans liquid. Its high heat capacity, which makes it a thermal buffer for the planet and for organisms. Its surface tension, which lets water climb a tree against gravity and lets insects walk on ponds. Its capacity as a solvent for anything charged, which is why biochemistry happens in it.

And the one that matters most is the strangest. Nearly every substance is denser as a solid than as a liquid, because cooling slows molecules and lets them pack closer. Water is not. When it freezes, the hydrogen bonds lock the molecules into an open hexagonal lattice with gaps in it, and ice is about 9% less dense than liquid water.

So ice floats. Lakes freeze from the surface down, and the ice insulates what is beneath, and whatever is living in there survives the winter. If water behaved normally, ice would sink, lakes would freeze solid from the bottom, and the northern hemisphere would be considerably emptier.

That is a very short chain from a bond angle to the habitability of a planet.

Which requires the general picture underneath it, and one correction to how bonds are usually pictured.

A chemical bond is not a thing joining two atoms. It is a region where the arrangement of electrons lowers the total energy of the system compared with the atoms being separate. The stick in a structural diagram is bookkeeping. This matters because it explains why bonds have characteristic strengths and lengths (there is an optimal separation where the energy is lowest) and why breaking one costs energy rather than requiring some physical rupture.

Bonding is also a spectrum rather than two categories. The textbook division between ionic, where electrons transfer, and covalent, where they are shared, is really a question of how unequally the sharing goes. Sodium chloride sits near the ionic end; a hydrogen molecule is perfectly covalent; and most real bonds are somewhere in between, polar covalent, with a partial charge at each end.

Those partial charges are what produce hydrogen bonding, protein folding, the shape of DNA and the behaviour of every solvent. The interesting chemistry lives in the middle of the spectrum, not at its ends.

Which brings us to carbon, and to why it rather than something else.

Carbon has four bonding electrons, which allows chains, branches and rings. Carbon-carbon bonds are strong enough to be stable at the temperatures life operates at, and weak enough to be broken by the energies life has available: a narrow window. And its bonds to hydrogen, oxygen and nitrogen are of broadly comparable strength, so complex molecules can be assembled and disassembled at reasonable cost rather than being locked permanently in whatever form they first took.

Silicon, directly below carbon, also has four bonding electrons, which is why silicon-based life is a staple of speculation. Silicon-silicon bonds are much weaker, long silicon chains fall apart, and silicon dioxide is a solid rather than a gas, so a silicon organism would have to exhale sand.

One last consequence of three-dimensional bonding that turned out to matter enormously.

A carbon with four different groups attached can exist in two forms that are mirror images and cannot be superimposed, like a pair of hands. They have identical formulas, identical bonds and identical physical properties in most respects.

Biological receptors are built from amino acids that are themselves chiral, so a receptor fits one handedness and not the other, exactly as a right glove fits one hand. This is why one enantiomer of a drug can be therapeutic and the other inert, or worse. Thalidomide is the case everybody knows, where one form was a sedative and the other caused severe birth defects, and the tragedy is compounded by the fact that the forms interconvert in the body, so separating them beforehand would not have prevented it.

It is also why every amino acid in every living thing is left-handed and every sugar is right-handed, which is one of the strongest single pieces of evidence that all life on Earth shares one origin.