Molybdenum Fixes Nitrogen

Molybdenum's odd-sounding name comes from its resemblance to lead ores. That makes three elements: plumbum (lead), carbon (plumbago), and now molybdenum named after lead. Perhaps it's not so much the color as the texture:

Native ore: MoS₂

Native plumbago (graphite)

Native lead (Galena, PbS)

Molybdenum's role in life is anything but leaden. Mo is in nitrogenase, an enzyme absolutely essential for life. Nitrogenase-containing bacteria "fix" nitrogen, turning thin air into solid plant food (ammonium), a feat no less remarkable than photosynthesis. The bacteria in turn get first dibs on the sugars made by the plants in perfect symbiosis.

If artificial photosynthesis is cool--making sugars and fuels from CO₂--nitrogen fixation is even more exciting because we're about a third of the way there.*  Bacteria make about 2/3 of the world's fixed nitrogen, and the ancient Haber-Bosch process does the rest.  The Haber-Bosch process, first developed in 1913, enabled Germany to fight the First World War. Today it helps feed about a third of the world's population but it also consumes about 1-2 % of the world's annual energy supply. We can do even better, perhaps by learning from nature.

Nitrogenase manages to fix nitrogen under much milder conditions than Haber-Bosch, though like many natural processes, the yield is diffuse and unusable industrially. Molybdenum's connection to nitrogen fixation was first noted in 1930 when bacteria raised on ammonia showed no need for the element while those raised on nitrogen did (before the enzyme nitrogenase had even been identified). When nitrogenase was found, the next step was to crystallize it and determine its structure. This happened in 1992, at a time when molybdenum and the heavier tungsten were already famous for loosely binding things like H₂ and N₂ in synthetic tungsten compounds. Some thought perhaps nature had gotten there first. Alas, the structure gave few clues as to where and how nitrogen is activated, except that it happened stepwise, using protons and electrons instead of H₂.

Another really cool thing about the nitrogenase X-ray crystal structure was the revelation of a mystery element "X" which could be either a C, an N, or an O atom. Several contact points surround the mystery element X which makes that a "Texas" atom.  More recent work, published last year in Science, says it is carbon (link). More on that later.
______________
*There are vanadium-dependent nitrogenases which convert CO to alkanes--just like Fischer-Tropsch catalysts do: link  That would be so cool if the X-factor were a Texas carbide.

Conversations with Henry

Henry:  I once gave an exam by passing out a blank Periodic Table and asked students to fill in the blanks from memory:

Me: Yikes! Did anybody pass?

Henry: A few. But everyone got vanadium right!

Vanadium Adds Color

Vanadium is the first element of color. The previous element, titanium, is actually the essence of butt-white: Titanium dioxide, TiO₂ is commonly used in white paint and also as a sunscreen. Of all the other previous elements, only chlorine is a pretty lime green. I suppose that sulfur counts as yellow and boron counts as brown, and certain allotropes of carbon counts as black--but only as solids. By "element of color" I mean that vanadium is the first of many elements that form pretty colored water solutions:
 

Oxidation states of vanadium, from left +2 (lilac), +3 (green), +4 (blue) and +5 (yellow).

There's a reason why vanadium does this, but I'm going to hold off a simplified explanation until I get to chromium (nominally the best example).

Vanadium's discovery is a bit sad in that its true discoverer, a fellow named del Rio, was talked out of his discovery by a Frenchman. Our old friend Friedrich Wöhler showed up and straightened things out, but not before the Swedish name stuck. The whole naming history is here.

Vanadium has an oxidation state of +5 in the oxide VO₅.  VO₅ has nothing to do with the shampoo VO5, but I thought that the first photo bore such a striking likeness to the second one that I put it here just to confuse things.

In the Periodic Table, vanadium belongs to Group 5. Chemistry students recognize the convenient mnemonic for vanadium (V is five in Roman numerals). 

Most of the world's mined vanadium goes into steel where it alloyed with iron to improve its strength. Vanadium also has some very interesting biological properties and may even be a micro nutrient essential to humans (it is essential to rats and chickens). It also appears to have promise in high-tech batteries.