CHE 101 Lecture Notes - Lecture 5: Henri Becquerel, Pierre Curie, Alpha Particle

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The Rutherford Bohr Model of the Atom
Within a short time after Thomson published his model, a fairly detailed and accurate picture
of the atom’s structure emerged. Even as Thomson was investigating the atom, other workers
were finding other pieces of the atomic puzzle, although where the pieces fit was not
immediately clear.
Henri Becquerel was trying to study light emission by minerals when he found that energy
was given off by uranium-containing minerals, even when they were not exposed earlier to
light. He had accidentally discovered radioactivity, that is, the ability to spontaneously radiate
energy.
Marie and Pierre Curie followed up on Becquerel discovery and separated various radioactive
elements and studied their properties. They coined the term radioactive decay.
The greatest contribution to the study of the atom was made by Sit Ernest Rutherford, a New
Zealander who worked for nine years making fundamental discovery after discovery in
Montreal before moving to Manchester, England. The following summary of his work
reveals why he was awarded the Nobel Prize in chemistry in 1908.
a. Discovered the alpha, beta, and gamma particles. Initially, Rutherford found two new types
of particles. One particle was easily stopped by a thin piece of metal-he called this alpha
particle. Beta particles eventually were found to be high energy electrons emitted directly
from the atomic nucleus. He later detected gamma particles and found them to be high energy
radiation given off by the nucleus.
b. Correctly interpreted the nature of radioactivity. He showed that new elements were
formed as a result of the radioactive decay of an element.
c. Discovered the true nature of the alpha particle. He showed that a particles were simply He
ions.
d. Discovered the atomic nucleus. When he bombarded a piece of thin gold foil with alpha
particles, most of the particles went straight through, which meant that atoms were mostly
empty space. A few of the heavy alphas particles bounces straight back, which meant that
there was a very tiny region in the atom where most of the mass was concentrated: the
nucleus. When expressed as a multiple of the charge to the atomic number of the atom. He
postulated that the nucleus contained all the protons and most of the mass of the atom.
e. Predicted the existence of the neutron. As soon as Rutherford found that the atomic nucleus
contained protons, he had a serious problem. Protons have a mass of 1 and a charge of +1.
The helium nucleus was known to have a total charge of +2, which implied that it contained 2
protons, but it also had a mass of 4. He therefore suggested that the nucleus must contain
neutral particles having a mass of 1, so as to account for the extra mass of He.
In 1932, J. Chadwick discovered the neutron predicted by Rutherford.
Rutherford’s work was concerned with the atomic nucleus and was not overly concerned with
what the electrons were doing. Once he found that electrons had to exist outside the nucleus,
others had to deal with the problem of electron behavior. The planetary model of electron
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