Physical Science: Session 4
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.
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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