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Module 1: Los orígenes de los elementos

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Los elementos naturales

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XSIQ
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Chemistry - The naturally-occurring elements

The naturally-occurring elements

It is currently believed that the universe began with the primordial event
known as the an extreme explosion in which super-dense, super-hot matter
expanded rapidly to produce mainlyor protons. In the first few minutes of
this event the temperatures remained high enough for the fusion of hydrogen
nuclei to produce helium nuclei. Approximately 99 per cent of the atoms in
the universe were created during the big bang. As temperatures dropped over
time, the H and He nuclei were able to attract electrons and become H and
He atoms.

All elements heavier than hydrogen and helium were produced in stars.
Stars may well be considered the most important component of the universe.
Life as we know it depends on from our nearest star, the sun.

A star forms when a cloud of hydrogen gas contracts under its own gravity.
As the cloud of hydrogen contracts, theHowever, since the total energy of
the system must remain constant, the decrease in potential energy is
accompanied by an of the hydrogen atoms. The increasing kinetic energy
manifests itself as increasing temperature. Eventually the hydrogen atoms
become ionised into protons and electrons in a situation known as the
plasma state.

If the temperature in the star rises to 10 K (10 MK), the kinetic energy
of hydrogen nuclei is high enough to overcome their electrostatic repulsion
and they fuse together to produce helium nuclei.

A positron is a particle with the but with a Note that this equation is
effectively balanced with respect to both atomic number and mass number.

In our sun, and stars of similar mass, only He is produced by nuclear
fusion or, as it is often described, hydrogen burning. Our sun is simply
too small to generate the temperatures necessary to produce elements
heavier than helium. Hydrogen burning produces lots of energy, as we know
from the heat and light generated by our sun.

Generating the temperatures of around 10 K (100 MK) that are necessary to
initiate nuclear attraction between helium nuclei requires stars
approximately 1.5 times the mass of our sun.

As helium nuclei combine to produce larger nuclei, more energy is
released. When the temperature reaches 600 MK, neon nuclei become involved
in nuclear reactions and magnesium and silicon nuclei are produced.

At around 1500 MK, the reactions become even more complex. The various
nuclei that have collected in the different layers of the star combine,
producing nuclei of all the elements between Na and Ca.

At around 2000 to 3000 MK, in pretty massive stars, nuclei of the iron
group elements, first row transition series, are formed.

Elements heavier than iron are formed on the way to and during supernovas.
It is during supernovas that elements are ejected into space.

What happens to the elements after a supernova? Well, that's another
story! But it is fair to say that all the naturally-occurring elements on
Earth were at some point in their history ejected into space during
supernovas.

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