Guest author, Thomas Kennedy, features a twice-monthly series, Quantum Leap, wherein he guides readers through the fascinating world of quantum mechanics. This is issue 013.
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*This issue is continued from Issue 012, “Uncertainty“

Note: Just as no map can equal a territory, no concept of an atom can possibly equal its nature.   These models of the atom simply served as a way of thinking about them, albeit they contained limitations, as all models do.

Although the mathematical concept of the atom got better, the visual concept of the atom got worse.  Regardless, even simplistic visual models can still prove useful.  Chemists usually describe the atom as a simple solar system model similar to Bohr’s model, but without the different orbit shapes.  The important emphasis for chemistry medium attempts to show the groupings of electrons in orbital shells.  (The example above shows the first eleven elements.)

Chemical behavior of the elements form together to create molecules.  Molecules may share electrons as the hydrogen and water molecules above illustrates.  (Atoms which share electrons have the name “ions.”)  The outer electron shell of an atom actually does the sharing and bonding of the atoms.  This in turn allows chemists to describe the interactions of chemistry.  Even though the orbit model of the atom does not provide an accurate model, it works well for describing chemistry.

(A helium atom with two electrons orbiting a nucleon made of two protons and two neutrons)

A mystery of the nature of the nucleus remained unsolved.  The nucleus contains most of the atom’s mass as well as the positive charge.  The protons supposedly accounted for this mass.  However, a nucleus with twice the charge of another should have twice the number of protons and twice the mass.   But, this did not prove correct.  Rutherford speculated in 1920 that there existed electrically neutral particles with the protons that make up the missing mass, but no one accepted his idea at the time.

Not until 1932 did the English physicist, James Chadwick, finally discover the neutron.  He found it to measure slightly heavier than the proton with a mass of 1840 electrons and with no charge (neutral).   The proton-neutron together received the name, “nucleon.”

(Isotopes of Hydrogen)

Although scientists knew that atoms of a particular element have the same number of protons, they discovered that some of these atoms have slightly different masses.  They concluded that the variations in mass result, more or less, from the number of neutrons in the nucleus of the atom.  Atoms of an element having the same atomic number, but different atomic masses, are called “isotopes” of that element.

*Thanks to the work of Jim Walker for his contribution to this series.

Next and final article in the series: You are so negative …

Watch for Issue #14 of Thomas’ “Quantum Leap”, on May 28, 2010.

You can access all previous issues of “Quantum Leap”, here.

Tags: atom, Bohr, isotopes, James Chadwick, molecules, nucleon

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