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In a book I am reading, it is said

In the $\ce{HCl}$ molecule, the shared pair of electrons spend more time nearer the chlorine atom. In the $\ce{HF}$ molecule, the shared electrons spend more time nearer the F atom.

What does "spend more time" mean here? Has this been experimentally validated? If yes, how?

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When you come to the quantum level (looking at one electron) things don't behave anymore like we normally think. To make it anyway understandable those expressions were invented such as "spending more time". Electrons are less described as a dot but as a smearing (electron density). Chlorine and fluorine have higher electronegativities than hydrogen. The electron density therefore is not equally distributed between the two bonding partners but drawn towards fluorine/chlorine. To illustrate this, you can say "the electron spends more time at F/Cl".

This thinking was established as molecular orbital theory (s orbitals, p orbitals, ...). There have been other models before, e.g. you might know the shell model (k shell, l shell, ...). For experimental evidence you might want to take at look at this -- but most of all the citations therein: http://en.wikipedia.org/wiki/User:Chem507f10grp4/sandbox#Experimental_Evidence_Supporting_Molecular_Orbital_Theory Basically, the shell model ran into a series of properties it did not calculate correctly or precisely. Molecular orbital theory was found to predict many properties and came to well calculated values.

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    $\begingroup$ There is now a technique that can visualize chemical bonds vis a vis their electron density and how that electron density interacts with a carbon monoxide molecule on the tip of an atomic force microscope. rsc.org/chemistryworld/2013/09/… $\endgroup$ – Ben Norris Jan 23 '15 at 11:15
  • $\begingroup$ It would also be notable to say that what the textbook said was to clarify the meaning of electronegativity, along with MO basics. $\endgroup$ – M.A.R. Jan 23 '15 at 11:16

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