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Hydrochloric Acid: Bonding and PropertiesA Hydrochloric acid molecule consists of a Chlorine atom and a Hydrogen atom sharing one electron each so that they both have a full outer shell. Hydrochloric acid molecules can be understood easily if we look at the electron dot diagrams of each of the atoms involved: 
Forming HCl 
The molecule is made by joining up the Hydrogen and Chlorine atoms so that they share 1 pair of electrons. This gives each of the atoms a full outer shell of electrons and a degree of stability as shown in the diagram to the right. The pairs of electrons that Chlorine possesses that are NOT bonded to anything are also important and cannot be excluded. These are called lone pairs as they are electron pairs, but are not bonded to anything, ie they are alone.The last stage of the diagram represents Hydrochloric acid, also correctly called Hydrogen Chloride, in the standard way. Each pair of electrons is replaced with a single line. Actual HCl molecules are not flat but are three dimensional. The electron pairs are arranged around the Chlorine atom in a tetrahedral fashion which gives the same molecule regardless of which electron pair the Hydrogen atom is attached to. This follows the Valence Shell Electron Pair Repulsion theory, or VSEPR. The Electronegativity of Chlorine and Ionization The sharing of the electrons in the H-Cl bond is not even. This is due to the high electronegativity of Chlorine, which results in the Chlorine nucleus pulling the electrons in that pair closer to it than does the Hydrogen atom. In fact, the electrons from that bond spend virtually no time around the Hydrogen nucleus. This means that the Hydrogen nucleus, which is just a single proton, is very loosely held to the Chlorine atom. As soon as the HCl molecule encounters any other atom or ion that has even a slight tendency to pull positively charged particles (such as protons) towards it, the proton will detach and so the HCl molecule will break up into a free proton (+ charge) and a negatively charged Chloride ion: 
The ease with which HCl into H+ and Cl- ions is what defines HCl as a strong acid. This has nothing to do with the concentration of the acid. In solution, when HCl is mixed with water, the acid ionizes instantly and the free proton then attaches itself to one of the lone pairs on a water molecule, forming the Hydronium ion which has the chemical formula H3O+. Return from Hydrochloric Acid to Chemistry Definitions or return to the Green Planet home page for more Solar Power Facts.
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