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A Closer Look: Are All Atoms the Same?
Although Dalton was able to make correct predictions about the phenomena
he observed in the 1800s, one of his main assumptions — that all
atoms of a given substance are identical — was not correct, based
on what we now know about the subatomic structure of the atom.
John Dalton.
While an atom is the smallest particle
of an element that retains the intensive properties of that element,
all 92 stable atoms are made of
only three
smaller, more fundamental particles: the proton, the electron, and
the neutron. A simple model of the atom (refined by physicist Neils Bohr
in 1922) shows the protons and neutrons packed together in the core
of
the
atom, called the nucleus, and the electrons orbiting in specific paths
around the nucleus. Although the development of quantum mechanics in
the early 20th century led to our modern understanding of the atom
as having
a more indefinite structure than Bohr described, most chemical interactions
among atoms are governed by the electrical charges of their electrons
and protons.
Why we can forgive Dalton, and what is an isotope?
Since Dalton was only
able to probe the chemical behavior of materials, he assumed all atoms
of the same element were identical. Recall that
neutrons, along with protons, make up the nucleus. Since neutrons
have no electrical
charge, they do not affect the chemical behavior. What Dalton didn’t
know is that there can be different numbers of neutrons in atoms
of the same element — that is, the atoms are not all identical.
Atoms of the same element with different numbers of neutrons are
called isotopes.
Why are the differences between isotopes important?
Although chemical behavior
is not different for most isotopes, the behavior of different isotopes
of the same element at very
high temperatures
and
pressures can vary. Nuclear fission reactions, which occur
in nuclear power plants and stars, offer a good example of ways the isotopes
can affect
an atom. For example, when a uranium atom with 92 protons and
143
neutrons (U^235) is bombarded with neutrons, it releases large
amounts of energy
in a fission reaction. However, a uranium atom with 92 protons
and 146 neutrons (U^238) will not produce a fission reaction
when bombarded
with
neutrons.
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