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Module 1: Historia y desarrollo de la teoría atómica

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Ernest Rutherford

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XSIQ
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Chemistry - Ernest Rutherford

Ernest Rutherford

Rutherford, a New Zealander, was one of the scientists who identified the
three types of radiation, that originated within a radioactive atom,
namely:

The emission of radiation by atoms particularly intrigued Ernest
Rutherford. He realised that a radioactive source such as radium could be
used to investigate the internal structure of an atom.

Having shown that alpha particles [1] were helium nuclei, i.e. helium
atoms with a double positive charge, He2+, Rutherford decided to use these
a-particles, which were ejected from radium at 20000 kilometres per second,
as probes to investigate the internal structure of an atom. In the famous,
a-particles were fired at thin sheets of gold foil. A fluorescent screen
was used to follow the path of the a-particles after they had hit the foil.

The golf-foil experiment.

The experiment was actually carried out by two of Rutherford's students,
Geiger and Marsden, using gold foil only about 1000 atoms thick. They
observed that while most of the alpha particles passed straight through the
foil without moving from their original path, some particles deviated from
the original path as they passed through the foil. About one in 8000
bounced back towards its source.

From these observations, Rutherford concluded that most of an atom is
empty space. He proposed a nuclear model of the atom that describes atoms
as consisting of:

* a very small central nucleus containing most of the mass and all the
positive charge

* a large space outside the nucleus occupied by the electrons but
containing very little of the mass.

Rutherford believed that the electrons moved around the nucleus like
planets around the sun.

Rutherford's nuclear model of the atom was an inspired move, and was
confirmed experimentally. The time was soon to come when the role of the
electrons would become significantly clearer.

It was Henry Moseley, one of Rutherford's students, who proposed that the
atomic number - the number of positive charges on the nucleus of an atom -
directly influenced the chemical properties of a particular element. Since
each atom would have a number of electrons equal to the number of positive
charges in the nucleus, the way these electrons were arranged would
eventually be shown to influence the way an element behaves in chemical
reactions.

Moseley was able to confirm that the number of positive charges in the
nucleus of any atom equals its numerical position in the periodic table.
Atomic number, rather than atomic weight as used by Mendeleev, became a key
reference point for the arrangement of the elements.

Rutherford's of the atom was an inspired move, and was confirmed
experimentally. The time was soon to come when the role of the electrons
would become significantly clearer.

It was Henry Moseley, one of Rutherford's students, who proposed that the
- the number of positive charges on the nucleus of an atom - directly
influenced the chemical properties of a particular element. Since each atom
would have a number of electrons equal to the number of positive charges in
the nucleus, the way these electrons were arranged would eventually be
shown to influence the way an element behaves in chemical reactions.

Moseley was able to confirm that the number of positive charges in the
nucleus of any atom equals its numerical position in the periodic table.
Atomic number, rather than atomic weight as used by Mendeleev, became a key
reference point for the arrangement of the elements.

Rutherford's model of the atom still left plenty of room for
investigation.

The full structure of the nucleus was uncertain.

Why the protons in the nucleus didn't repel each other was not clear.

Why the orbiting electrons did not spiral into the nucleus as a result of
electrostatic attraction had not been explained.

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