Basic Structure of Atom

The Atom and Its Fundamental Structure

Hello,

This post is the start of the "Structure of Matter" page.  Let's begin with the basic #structure of the atom.  The #atom is the smallest unit of matter that retains the properties of elements.  An atom of an element cannot be broken down into atoms of lighter elements by any chemical process (reaction).

Ernest Rutherford's Gold Foil Experiment

A true understanding of #atomic structure began in 1907 with the published results of Ernest #Rutherford's Gold Foil Experiment.  The purpose of this experiment was to establish the locations of #electrons and #protons within the atom.  Prior to 1907, it was commonly believed that electrons and protons were evenly distributed throughout the confines of the atom.  This even-distribution theory began with J. J. #Thomson's 1897 experimental confirmation of the electron's existence and came to be known as The #Plum-Pudding Model of the Atom.

The #Gold Foil Experiment consisted of an #alpha-particle generator aimed toward the center of a very thin sample of gold foil (only 2-3 atoms) thick.  A long strip of paper impregnated with zinc sulfide served as an alpha-particle detector.  The alpha-particle detector's color would fade to white at locations where particles crossed and was wrapped completely around the thin gold foil.  The diagram below shows the lay-out of the gold foil experiment and indicates the results which Rutherford observed.

As indicated above, most alpha #particles went clear through the gold foil.  Because the alpha particle is the most massive radioactive particle (i.e. a helium atom nucleus), only the relatively massive (and positively charged) protons could stop them.  A proton is about 1800 times heavier than an electron.  Gold atoms contain many protons (79 in fact), so if the Plum-Pudding model was correct the alpha particles would very likely encounter protons and get deflected back.  Rutherford concluded that only gold atoms with a relatively very small cluster of protons located at the atomic center could allow most alpha particles through and yet still sharply deflect a few.

And so, the new #theory of atomic structure (small and massive proton cluster at the center with the surrounding space occupied by electrons) was born and has held to our time (with one addition)!  The results of the gold-foil experiment also indicated that the mass of a gold atom was about double that which could be explained by the protons alone.  Rutherford assumed that their must be another neutral particle clustered with the protons to make up the "missing" mass.  He called this particle the #neutron.

In my next post, I'll discuss the discovery of the neutron particle and the existence of #isotopes.

Bye for now!