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How does dissolving a salt molecule in water make its atoms ionize?
Category: Chemistry
Published: September 23, 2013
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and Assistant Professor of Physics at West Texas A&M University
Dissolving a salt molecule in water does not make its atoms ionize. The atoms in solid salts are already ionized long before touching water.
Electrons in an atom can only take on specific wave states, and only one electron can occupy one wave state at a time. As a result, electrons in an atom take different states, starting from the lowest energy state and going upwards in energy until the electrons have all found distinct states. For various reasons that are not worth mentioning here, electron states in atoms tend to form various groups, with the states in the same group having very similar energies and states. Chemists call these groups of electron states "shells", even though they have nothing to do with literal shells.
The interesting thing is that an atom with completely filled shells is very stable (all the available states in each group are occupied by electrons). On the other hand, an atom with its outermost shell only partially filled has a strong tendency to steal, lose, or share electrons from other atoms in order to fill its outermost shell and become stable. Such atoms are therefore chemically reactive. A well-known salt is sodium chloride (table salt), so let's use it as an example. A single neutral sodium atom has eleven electrons. Ten of these electrons fill states such that they form complete shells. The eleventh electron of sodium, however, is alone in the outermost, partially filled shell. Electrons are bound in atoms because their negative electric charge experiences electric attraction to the positive charge of the atom's nucleus. But for sodium, the negatively-charged electrons in the inner, completed shells do a good job of blocking, or screening, the attractive force of the nucleus on the eleventh electron. As a result, the eleventh electron of sodium is loosely bound to the atom and is ripe for being stolen by a more powerful atom.
In contrast, chlorine (17 electrons) has all of its shells filled with electrons except for its outermost shell which is one electron short of being complete. There is a very strong attraction by the chlorine atom on an outside electron which is needed to complete its shell. Sodium and chlorine are therefore a perfect match. Sodium has one electron it is not holding onto very strongly, and chlorine is looking for one more electron to steal to fill its shell. As a result, a pure sample of sodium reacts strongly with a pure sample of chlorine and the end product is table salt. Each chlorine atom steals an electron from the sodium atom. Each sodium atom now has 11 positive protons and 10 negative electrons, for a net charge of +1. Each chlorine atom now has 17 positive protons and 18 negative electrons for a net charge of -1. The atoms have therefore been ionized by the reaction that forms solid table salt, all without the presence of water. Both the sodium and the chlorine ions now have completely filled shells and are therefore stable. This is a good example of an atom that naturally has an unequal number of electron and protons.
The net positive sodium ion is now attracted to the net negative chlorine ion and this attraction forms what we call an "ionic bond". But, in reality, we don't have just one sodium ion sticking to ion chlorine ion. Instead, a lattice of many sodium ions ionically bonds to a lattice of chlorine ions, and we end up with a crystalline solid. Each sodium ion in the crystalline lattice of table salt is bound to the 6 nearest chlorine ions, and the same goes for each chlorine ion. The atoms in table salt are therefore already in the ionized state.
Adding water does not ionize the atoms in salt, because they are already ionized. Instead, the water molecules stick to the already formed ions in the salt. The textbook titled Cell and Molecular Biology: Concepts and Experiments by Gerald Karp states, "A crystal of table salt is held together by an electrostatic attraction between positively charged Na+ and negatively charged Cl– ions. This type of attraction between fully charged components is called an ionic bond (or a salt bridge). Ionic bonds within a salt crystal may be quite strong. However, if a crystal of salt is dissolved in water, each of the individual ions becomes surrounded by water molecules, which inhibit oppositely charged ions from approaching one another closely enough to form ionic bonds." Each water molecule has a permanent dipole, meaning that one end is always slightly positively charged and the other end is always slightly negatively charged. The charged ends of the water molecules are so strongly attracted to the charged ions in the salt crystal that the water destroys the solid lattice structure of the salt and each sodium and chlorine ion becomes surrounded by a layer of sticky water molecules. In chemistry, we say the salt has been dissolved by the water. It's like a rock band exiting the limousine into a crowd of fans and becoming separated as each band member gets surrounded by his own circle of fans. If the atoms in solid salt were not ionized to begin with, the water would not do such a good job dissolving the salt.