The Ionic Bond

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(say in a deep, suave voice ala Sean Connery) My name is Bond, Ionic Bond. Here's the audio.

Historical Introduction

In the same paper in 1897 that J.J. Thomson announced the discovery of the electron, he also speculated briefly on the electron's role in chemical bonding. He writes:

"There seems to me to be some evidence that the charges carried by the corpuscles in the atom are large compared with those carried by the ions of an electrolyte. In the molecule of HCl, for example, I picture the components of the hydrogen atoms as held together by a great number of tubes of electrostatic force; the components of the chlorine atom are similarly held together, while only one stray tube binds the hydrogen atom to the chlorine atom."

Later in the same paragraph, Thomson discusses the fact that one end of the "bond" (he hasn't used that word yet) is positive and the other negative.

"Wait a minute," you might say. "If negative charges could be transfered, why not positive ones also?" Glad you asked. About the same time that Thomson was doing his work, Wilhelm Wein was studying positively charged ions in gas discharge tubes. (Gas discharge tubes make both cathode rays -- what Thomson was studying -- and canal rays -- what Wein was studying. You set the tube up differently, depending on which one you want to study.)

Wein concluded that the positive charges did not exist independently of matter or in the "free state." This work supported the idea that positive charges (unlike negative) were not readily transferred from one atom to another and so could not be involved in bonding.

From that time on, bonding theories involving the transfer of electrons became the most widely theory and many people in America and Europe made contributions.

J.J. Thomson (in 1904) in Electricity and Matter, writes:

If we interpet the "bond" of the chemists as indicating a unit Faraday tube, connecting charged atoms in the molecule, the structural formulae of the chemist can be at once translated into the electrical theory . . . but the symbol indicating a bond on the chemical theory is not regarded as having direction, no difference is made on this theory between one end of the bond and the other. On the electrical theory, however, there is a difference between the ends, as one corresponds to a positive, the other to a negative charge. (p. 134)"

This theory had two problems:

  1. It failed to explain the bonding in non-ionizing substances. It only explained substances which formed ions in solution.
  2. It failed to explain the difference between strong and weak electrolytes.

The first problem would be solved with the covalent bond and the second when it was realized that strong electrolytes ionize 100% in solution.

Which elements form ionic bonds?

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