covalent bonds

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epsilonprodigy

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My MCAT book uses HCL as an example of a polar covalent bond. How is it covalent if it is able to dissociate into H+ and Cl- ions? What exactly defines a covalent bond then?

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HCl + H2O-> H3O+ + Cl-

So, it doesn't really just dissociate; it reacts with water, producing hydronium ion. Treating the reaction as if H+ is produced is a simplification.
 
You're confusing 2 different concepts.

Dissociation is affiliated with how strong/weak an acid/base is. Intermolecular forces can tell you whether or not the molecule will dissociate or not.
 
The difference in electronegativity between the two atoms in question determines the type of bond.
 
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I agree it is a sort of trick question because it makes you think of something like NaCl which exists solely as ions. But that is not the case with HCl. H-Cl has a very very high dissociation constant (I think ~10^6), but that doesn't mean there won't be ANY H-Cl molecules in solution. You may have also come across HCl gas before, and that is not ionic at all.
 
The net dipole moment defines the electronegativity difference between two atoms. Given a molecule with only two atoms, one need only about the electronegativity difference between the atoms. I've seen two thresholds: 1.6 and 2.0. If the electronegativity difference between two atoms in a binary molecule is 0, then the bond is nonpolar covalent: the electrons are equally shared. If the electronegativity difference between two atoms in a binary molecule is greater than 0 but less than 1.6 or 2.0 (I've seen both of these thresholds quoted), then the bond is polar covalent, and the atoms are shared unequally, favoring the more electronegative molecule. If the electronegativity difference between two atoms in a binary molecule is greater than 1.6 or 2.0 (depending on the threshold employed), then the bond is ionic, and the atoms are said to "gain or lose" electrons.
 
The net dipole moment defines the electronegativity difference between two atoms. Given a molecule with only two atoms, one need only about the electronegativity difference between the atoms. I've seen two thresholds: 1.6 and 2.0. If the electronegativity difference between two atoms in a binary molecule is 0, then the bond is nonpolar covalent: the electrons are equally shared. If the electronegativity difference between two atoms in a binary molecule is greater than 0 but less than 1.6 or 2.0 (I've seen both of these thresholds quoted), then the bond is polar covalent, and the atoms are shared unequally, favoring the more electronegative molecule. If the electronegativity difference between two atoms in a binary molecule is greater than 1.6 or 2.0 (depending on the threshold employed), then the bond is ionic, and the atoms are said to "gain or lose" electrons.
Great explanation...
 
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