Fleming and his colleagues created artificial isotopes of hydrogen with greater mass differences to rigorously test quantum theory. They made an ultra-light isotope by replacing a proton with a muon, which has 11% of a proton's mass. They made an ultra-heavy isotope by replacing an electron in helium with a muon, which orbits closer to the nucleus and effectively makes it hydrogen-like. They found the reaction rates calculated from quantum theory closely matched experimental measurements, giving confidence in theoretical methods for more complex systems.

1. Artificial hydrogen tests quantum theory
Heavy and light analogues of hydrogen probe the limits of quantum chemistry.
Philip Ball (Fleming, D. G. et al. Science 331, 448-450 (2011))
A normal hydrogen atom contains a single negatively charged electron orbiting a nucleus made of a single positively
charged proton. About 0.015% of natural hydrogen consists of the heavy isotope deuterium, in which the nucleus
contains a proton and an electrically neutral neutron, and which has a mass twice that of normal hydrogen. And there is
a third isotope with a proton and two neutrons: tritium, three times as massive as hydrogen, which is produced in trace
quantities by cosmic rays interacting with the atmosphere, but is too dangerously radioactive for use in such
experiments.
The chemical behaviour of atoms depends on the number of electrons they have rather than their masses, so the three
hydrogen isotopes are chemically almost identical. But the greater mass of the heavy isotopes means that they vibrate at
different frequencies, and quantum theory suggests that this will produce a small difference in the rates of their chemical
reactions.
To rigorously test that theory, isotopes of hydrogen are needed with
greater differences between their masses. Fleming and his colleagues
created some, using muons produced by collisions in the TRIUMF
particle accelerator in Vancouver.
Muons have many properties similar to electrons, but are more
massive. "A muon is an overgrown electron — an electron on steroids
— with a mass about 200 times that of an electron," says Richard Zare,
a physical chemist at Stanford University in California. "But unlike the
free electron, the free muon falls apart, with a mean lifetime of about
2.2 microseconds." This meant that the researchers had to work fast to
study their pseudo-hydrogen.
To make the ultra-light isotope, they swapped the proton with a
positively charged muon, which has just 11% of the mass of a proton.
And to make ultra-heavy hydrogen, they replaced one of the electrons
in a helium atom with a negative muon.
Helium has two electrons, two protons and two neutrons. But because
it is more massive than an electron, the negative muon orbits the
nucleus much more closely, masking the positive charge of one of the
protons. In effect, the atom becomes a hydrogen-like composite: a
'nucleus' made of the existing two-proton, two-neutron nucleus and the muon, orbited by the remaining electron. It has a
mass of a little over four times that of hydrogen.
Fleming and colleagues found that the reaction rates for hydrogen exchange involving these analogues that were
calculated from quantum theory were close to those measured experimentally. "This gives confidence in similar
theoretical methods applied to more complex systems," says Fleming.
Unexpected agreement
The close match between experiment and theory wasn't necessarily to be expected, because quantum calculations use
a simplification called the Born–Oppenheimer approximation, which assumes that the electrons adapt their trajectories
instantly to any movement of the nuclei. This is generally true for electrons, which are nearly 2,000 times lighter than
protons. But it wasn't obvious that it would hold for muons, which have a tenth of the proton's mass.
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"It surprises me at first blush that the theoretical treatments hold up so well," says Zare. "The Born–Oppenheimer
approximation is based on the small ratio of the mass of the electron to the mass of the nucleus. Yet suddenly the mass
of the electron is increased two-hundred-fold and all seems to be well."
Because the muon has such a short lifetime, extending such studies to more chemically complex systems is very
challenging.

2. Vocabulario
Overgrown: sobrecrecido close: cercano/a
But unlike: Pero a diferencia de Match: correspondencia
Fall apart: Deshacerse At first blush: A primera vista
Swap: intercambiar ratio: proporción, relación
Mask: enmascarando, ocultando Challenging: Desafiante, estimulante.
Cuestiones
1. ¿Qué dice el artículo que es un muon?
2. ¿Cuales son los isótopos naturales del hidrógeno?
3. ¿De qué está compuesto el isótopo artificial ultraligero del hidrógeno?
4. ¿De qué está compuesto el isótopo artificial ultrapesado del hidrógeno?
5. ¿Por qué el isótopo ultrapesado aunque es un átomo de helio se asemeja a uno de hidrógeno?
6. ¿Para qué necesitan los científicos estos isótopos?
7. ¿Estos isótopos cumplen la teoría cuántica?
8. Según lo que dice el artículo ¿estos isótopos deberían cumplir la aproximación de Born-
Oppenheimer?
9. ¿Por qué dice el autor que los resultados de este experimento son inesperados?
10. ¿Qué te ha parecido este artículo?