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January 8, 2012

(Short Periodic Tale introductory animation)

The discovery of hydrogen can be attributed to three people – or one of three people depending who you like most – Boyle (the father of modern chemistry, author of ‘The Sceptical Chymist’ and bad speller); Cavendish (genius, gynophobic – Google it, and cousin of the Duchess of Devonshire – you know, Keira Knightly in that film); and finally, Lavoisier (scientist, established the metric system and guillotined in the revolution).

Boyle, in 1671, isolated a gas that was evolved when acid was poured on iron filings. Cavendish, in 1781, established that the gas was an element and not a compound or mixture but he thought it was phlogiston (expect supplementary blog on phlogiston). Lavoisier, in 1783, gave the gas the name hydrogen, meaning water creator.

Hydrogen was the first element to be created after the Big Bang and today 90% of all atoms in the universe are hydrogen. Hydrogen comes in three forms – 1H – Hydrogen, with one proton and one electron; 2D – Deuterium, with one proton, one electron and one neutron; 3T – Tritium, with one proton, one electron and two neutrons. Tritium is not so interesting so I am going to ignore it – considering it occurs in only trace amounts in the upper atmosphere this is not difficult to achieve.

Deuterium is chemically very similar to hydrogen but the neutron makes deuterium almost twice as heavy. The effect this has on its chemistry is subtle and affects bond length and bond energy. The differences compared to ordinary hydrogen are enough that deuterated water (D2O) or heavy water is slightly toxic to eukaryotic organisms – a 50% substitution of heavy water can cause death. Several goldfish and one mouse have made the ultimate sacrifice, although there are no recorded deaths, accidental or otherwise, of humans. Deuterium makes up only 1% of all hydrogen in the universe, considering you could drink around 5 litres of heavy water with no appreciable side effects (other then frequent trips to the toilet) it is unlikely you will encounter enough in your day to day life to cause you any problems.

This reminds me of a tale that went round while I was a student. The story went that the CIA had made a deuterated dog where all the dog’s hydrogen atoms were deuterium. How and, more importantly, why was never discussed. The dog was fed deuterated food and water to maintain its deuterated status. This is obviously bollocks, and not even the dog’s bollocks. Maybe a particularly geeky student overheard a conversation about a ‘heavy dog’ and got the wrong end of the stick. Maybe I spent too much time around the ether bottle.

Bizarrely the CIA’s dog, if it could be made, would be invisible to a MRI scanner. The dog would still be clearly visible to the naked eye so apart from confusing the staff operating the scanner I can see no advantage to this. Its the hydrogen atoms in our body that show up in MRI scans because the proton at the Centre of the atom behaves like a tiny magnet. Inside the scanner each tiny hydrogen magnet in our body aligns itself with the MRI’s huge magnetic field (just as two ordinary magnets will align themselves north-south if you bring them close together). A short pulse of radio waves is enough energy to knock the protons out of alignment. The MRI detects to energy emitted by the protons as they return to their aligned state and can use this to create an image. Deuterium, with a proton and neutron at the Centre of the atom, does not behave like a magnet and is therefore invisible to the MRI scanner.

If the periodic table is a family then hydrogen is the black sheep. The small runty one that turns up at every gathering but no one knows who they are really related to. There is a resemblance to the alkali metals (group 1), but then also the halogens (group 17), and in fact hydrogen doesn’t really look like any group. Maybe hydrogen’s mum was friendly with the postman. This is why hydrogen is often seen floating aimlessly above the rest of the periodic table. Without it’s solitary electron (which it is quite happy to give away) it becomes a proton which isn’t even an atom or an ion but a subatomic particle.

When hydrogen bonds to an alkali metal it behaves like a halide and when its bonded to a halide it behaves like an alkali metal. Hydrogen normally makes only one bond per atom but hydrogen isn’t normal so it also forms bridging bonds between two or more atoms called, unsurprisingly, hydrogen-bonds. These bridging bonds may be unusual but they are essential to life, allowing water (H2O) to be liquid at temperatures commonly experienced on this planet and holding DNA strands together.

Hydrogen is so ubiquitous that listing all its roles in the chemistry of our everyday life I would be here forever and I promised more blogs on other elements. One of the more well known and interesting roles has been in transport. Hydrogen, going around as it normally does as a pair of hydrogen atoms, is lighter than air, a property exploited for use in air ships. The highly flammable nature of hydrogen (or the huge amount of energy released when it forms a bond with oxygen if you look at it slightly differently) stopped its use in air ships after the now infamous Hindenburg disaster. In defense of those that built the Hindenburg many believe it was the skin of the air ship that burnt initially rather than the hydrogen.

Hydrogen has continued to be used as a fuel but in liquid form and for rockets. Liquid hydrogen and liquid oxygen are mixed and ignited to generate the huge amount of energy needed to launch craft away from Earth’s gravitational pull and out in to space . Due to hydrogen’s very small size and very light weight making hydrogen liquid and storing it is no trivial matter. Hydrogen becomes liquid at temperatures below -253 Celsius and has to be stored in very sturdy containers else the tiny molecules will leak out.

We now look to hydrogen to be the fuel of the future through fuel cells – combining hydrogen and oxygen to form water but using a catalyst to convert the energy released into useable electricity. Again, storage of hydrogen is one of the major problems that needs to be overcome. Many researchers have explored the properties of platinum and palladium which have the ability to store huge amounts of hydrogen – but that is a story for another blog post.

There is much more to hydrogen for example, a metallic liquid form of the element may form the core of distant planets, but there is simply not enough time or space here to explore them all. I hope this has fed your imagination and encouraged you to find out more.

See you next week for Helium.


Images by @SciCommStudios

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