01 February 2011

Hydrogen: Accept No Substitutes

A helium atom consists of a nucleus containing two positively charged protons and two neutrons, encircled by two orbiting electrons which carry a negative charge. A hydrogen atom has just one proton and one electron. Donald Fleming of the University of British Columbia in Vancouver, Canada, and colleagues managed to disguise a helium atom as a hydrogen atom by replacing one of its orbiting electrons with a muon, which is far heavier than an electron. _NS

The cloak and dagger world of atomic promiscuity is generally ignored at the macro-level, where "a hydrogen atom is a hydrogen atom." But in the atomic world things are not so simple. Atoms may have too many or two few of particular subatomic particles and still chemically react with each other. Sometimes particles that don't belong -- such as muons or anti-muons -- can substitute for electrons or protons to create exotic dancers imposter atoms capable of bonding with conventional atoms.
In a feat of modern-day alchemy, atom tinkerers have fooled hydrogen atoms into accepting a helium atom as one of their own. The camouflaged atom behaves chemically like hydrogen, but has four times the mass of normal hydrogen, allowing predictions for how atomic mass affects reaction rates to be put to the test.

...Because it is so heavy, the muon sits 200 times closer to the helium nucleus than the electron it replaces and cancels out one of the nucleus's positive charges. The remaining electron then behaves as if it were orbiting a nucleus with just one positive charge, just like the electron in a hydrogen atom. The difference is that the nucleus is 4.1 times heavier than normal.

Fleming and his colleagues used this "super-heavy hydrogen", to test the effects of mass on chemical reaction rates.

...Two hydrogen isotopes, containing one or two neutrons and with two or three times the mass of normal hydrogen, can be used to test this. An even heavier isotope, with three neutrons, exists but decays too quickly. Muonic helium, which has the same mass as this isotope, lasts long enough to react with a hydrogen molecule.

Fleming's team shot muons produced at the TRIUMF accelerator in Vancouver into a cloud of helium, molecular hydrogen and ammonia. The helium atoms captured the muons, then pulled hydrogen atoms away from the molecular hydrogen and bonded with them.

The team compared how long this took with the rate of the same reaction using normal hydrogen, and with a reaction rate recorded in 1987 when a type of ultra-light hydrogen, called muonium, was used. Chemists formed this by replacing the proton in a hydrogen atom with an antimuon, the muon's positively charged antimatter partner.

As expected, the reaction with the disguised helium was the slowest, followed by normal hydrogen, then the light hydrogen [muonium]. The rates perfectly matched predictions from quantum mechanical calculations led by Fleming's teammate Donald Truhlar of the University of Minnesota in Minneapolis _NS
Source Muonium

These exotic atoms and molecules do not last long, but for these experiments they only have to last long enough to perform the experiment and collect the data. Eventually, we may learn to incorporate exotic matter into long-lived commercial products, exhibiting unique properties which make them valuable.

The truth is, we have barely begun to comprehend the universe in which we live. As for all the other universes? Fuggiduhbowdit!


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“During times of universal deceit, telling the truth becomes a revolutionary act” _George Orwell

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