The Stars and Us
Hydrogen is the smallest and lightest of all elements, but it plays a central role in physics, chemistry and biology. 74% of everything in the universe is hydrogen, by mass; it's more like 90% of all atoms, but each atom is very light, consisting only of one proton and one electron. Another 24% of the mass is helium, which is four times as heavy. In the early universe the proportions were even higher, but some of the hydrogen and helium has been used up in stars, making everything else in the universe. The 2% that isn't hydrogen or helium, in other words. Hydrogen is many stars' single biggest source of power, indeed it's the only source that smaller stars are able to use.
In chemistry, all standard acids contain hydrogen, and the main measure of acidity is pH, which is determined by the concentration of hydrogen ions (H+) in a solution. A hydrogen ion is an atom of hydrogen that's lost its one electron, leaving behind a single proton: it's a subatomic particle that will turn back into a real atom as soon as it manages to attract a new electron. Acids work because the hydrogen ion engages in a two-pronged attack along with the rest of the acid, which is always a negative ion. For every point that pH increases, the number of hydrogen ions goes down by a factor of ten, so something with a pH of 5 has ten times as many hydrogen ions as something with a pH of 6.
In biology, hydrogen ions are widely used for transporting and storing energy, playing a fundamental role in both respiration and photosynthesis. This 'proton pump' mechanism pushes the hydrogen ions from one side of a membrane to the other, and energy is released when they go back again, like a tiny boulder rolling back down a hill. There's more to it than that, but hopefully you get the gist.
The fact that hydrogen is prone to losing electrons and forming positive ions is one of the things that makes it rather odd, as an element. Usually, only metals do that. But hydrogen is an odd case in many ways; in chemistry teaching I often find myself saying '...except hydrogen.'
It's not only in the form of free protons that hydrogen is important, of course. Hydrogen is found in a huge range of compounds, including almost all organic compounds; it's so ubiquitous that some styles of representing organic compounds just leave out the hydrogen altogether. Instead, places where there isn't hydrogen, but there could have been, are marked out. We're left to fill in the gaps ourselves. That's what's meant by 'unsaturated', when people talk about fats and hydrocarbons - there's some hydrogen missing somewhere, so carbon atoms have to form double bonds with each other, making them less stable and frankly, a whole lot messier. Which is why unsaturated fats aren't solid at room temperature: they don't stack so neatly, so the molecules don't stick together as well.
The hydrogen contributes to the energy density of organic compounds, and having it there enables carbon to form stable chains, rings and so on. Left to its own devices, carbon would mostly only form graphite, diamond and fullerenes, which are all perfectly nice chemicals in their own way, but none of them is anywhere near interesting and versatile enough to give rise to the complex chemistry of life.
The most familiar of all hydrogen compounds is of course water (H2O). It's so much a part of everyday life that it's easy to lose sight of what an odd compound it is. It has the extraordinary property of getting bigger and less dense when it freezes, so unlike almost any other substance its solid form floats on top of its liquid form. If not for this anomaly, Earth's water would almost certainly freeze over, and life would never have had a chance to develop.
Even the fact that water is a liquid at all, at the temperatures we're used to, a little surprising. Most small molecules boil at far lower temperatures. The reason water doesn't has to do with the fact hydrogen is not very good at holding on to electrons, whereas oxygen has exceptional powers of electron-grabbing (it's electronegative, in chemist-speak). That makes for a very uneven molecular partnership, with the oxygen taking the lion's share of each electron for itself. So a water molecule consists of one oxygen atom with the negative charge of almost two whole extra electrons, and two near-naked protons hanging onto one side, with their positive charges exposed to the world. Opposite charges attract, so the hydrogens of one water molecule naturally tend to pull in the oxygen of another. This attraction, known as hydrogen bonding, is why water has such a high boiling point. It's also part of the reason why ice floats. Thanks to hydrogen bonding and the way the hydrogen atoms are arranged around the oxygen, for water to freeze the molecules need to line themselves up in a hexagonal honeycomb pattern, with one water molecule at each vertex, leaving a big gap in the middle.
Hydrogen bonding happens whenever you've got a hydrogen atom attached to oxygen or another electronegative atom, like nitrogen, fluorine or chlorine. It's the single strongest kind of intermolecular force, and it not only explains many of the properties of water, but also those of any other molecule where it features. So ethanol, the alcohol found in booze, is a liquid because it has a hydrogen atom attached to an oxygen (we call this a hydroxyl functional group); but it only has one, so it boils at a lower temperature than water. Glycerol, the alcohol that holds fat together, has three separate hydroxyl groups, so it's got a higher boiling point, and it's very viscous. Glucose has six hydroxyl groups, which is why it's solid at room temperature. All of these are soluble in water, and to some extent also in each other, because hydrogen bonds work between molecules of different types. The same kind of electrostatic forces make water the world's greatest solvent, allowing it to overcome the ionic bonds that hold together many crystals.
Hydrogen gas is of huge economic importance. The use with the most profound consequences for humankind is the Haber-Bosch process, which turns hydrogen and nitrogen into ammonia (NH3). Nitrogen fixation is such a limiting factor in growing food crops that without this one chemical reaction, we'd only be able to feed about half of the humans currently on this planet.
Producing hydrogen is straightforward, but unfortunately not very efficient. Any acid reacting with a metal will produce hydrogen bubbles, which is how they inflated dirigibles like the Hindenburg, but even the cheapest metals are not that cheap. Electrolysis can be used to split water into hydrogen and oxygen, but that takes quite a bit more energy than you'll ever get back out of it. So most industrially produced hydrogen is currently made from fossil fuels. The usual method is to heat methane with steam in the presence of a catalyst, producing carbon monoxide and hydrogen gas.
For decades people have been talking up the possibility of a hydrogen economy, in which fuel cells would take the place of petrol, and cars run on electrical power, emitting only water. If only we had plentiful hydrogen and good ways to store it and transport it, this would be a very appealing prospect. However, hydrogen is only a liquid at temperatures below about -252°C, and storing it in gas form is impractical unless it's under massive pressure. The challenges of maintaining such temperatures and pressures efficiently may yet be overcome, but given the competition from the developing technologies of algal fuels and battery power, each requiring far less infrastructure investment to take off, it seems likely that hydrogen cells will never fulfil their promise.
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