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Ionic Bonding

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In this lesson we'll look at one type of chemical bond, the ionic bond. Specifically we'll answer the following questions: What is an ion, how do ions form, and how do ions form ionic bonds? To answer these questions, we'll examine sodium chloride. Atoms join by ionic bonds to form ionic compounds, and sodium chloride is an ionic compound with many functions in our bodies. It forms from atoms of sodium and chlorine. You likely know it better as table salt. If you examine the label on packaged food items, this compound is typically listed just as salt. We'll start with sodium. Its atomic number is 11. By definition that means that a sodium atom has 11 protons. Recall that in a regular atom, the number of positive protons is balanced by an equal number of negative electrons. So if sodium has 11 protons, it has 11 electrons. Because chemical bonding involves only the valence electrons, I'll take a shortcut here and just put a ball in the center for the nucleus because I am far less concerned about the protons and neutrons that are in it than I am about the arrangement of the electrons around it. Recall that electrons organize very specifically. The first orbital can hold no more than two electrons, and then it's full. By the octet rule, the second orbital can hold eight more electrons. This gets us to 10, but sodium has 11, so we need a third orbital to accommodate that last electron. To be stable an atom must have a full outermost orbital. Is sodium stable? No, because it would need eight valence electrons but it only has one. Now let's look at chlorine. It's atomic number is 17, which means it has 17 protons and, thus, also 17 electrons. I'll draw the nucleus in the center for the protons and neutrons, then add the first orbital for the first two electrons and a second orbital for the next eight electrons. And this again puts me up to ten electrons, so I need a third orbital to accommodate the remaining seven electrons. Chlorine is also unstable because it has only seven valence electrons, not the eight needed for stability. Sodium has one lone valence electron, and chlorine is short one. Ideally, sodium would just give its valence electron to chlorine. That would get rid of sodium's single valence electron while filling chlorine's outermost orbital. And in fact that it was happens. Sodium's lone valence electron is attracted more strongly by chlorine's 17 protons than it is by sodium's own, but fewer, 11 protons. So sodium loses its lone valence electron and chlorine gains it. Watch. With that electron gone, the eight electrons in sodium's second orbital become its valence electrons, and chlorine also now has eight valence electrons. Both atoms now have full outermost electron orbitals, and, thus, each atom is stable. Yey! But we have a new issue. Each atom now has unequal numbers of protons and electrons, so they are no longer electrically neutral. We've just created two ions. By definition ions are atoms that have either lost or gained electrons, and, thus, because of the change in the number of electrons, ions carry an electrical charge. A positive charged ion is called a cation, and a negative ion is an anion. Here's a trick to keep them straight. The t in cation is like the plus sign, so it is positive. An anion has more n's, which helps me remember that anions are negative. Easy math helps us understand the electrical charges. Sodium started with 11 positive protons and 11 negative electrons but it lost an electron, so it now has only 10, giving it a net electrical charge of plus one. Thus sodium is a positive ion, or a cation. Chlorine started with 17 protons and 17 electrons but it gained an electron, giving it 18. So it now has a negative charge of minus one, thus making it a negative ion, or an anion. We designate ions by using the chemical symbol followed by a superscript plus or minus sign to indicate the electric charge. That gives us Na plus, or sodium ion, and Cl negative, which is also called chloride ion. Our atoms formed ions to become stable, but we don't yet have an ionic bond. That comes after the ions form for a real simple reason: opposites attract. Our ions have opposite electrical charges, and because of this, they are attracted to each other, moving close together so their charges balance out. This is an ionic bond, and in this case, the bond produces the compound sodium chloride. To summarize, ionic bonding requires two steps. First, atoms gain or lose electrons to become stable, thus forming ions. And then ions of opposite charges move close together so their charges are balanced. This closeness is the basis of the ionic bond through which ionic compounds form. Ionic bonds are relatively strong, but compounds held together by them can easily separate, or dissociate, in watery solutions such as our body fluids, freeing the ions. For this reason ionic compounds are what we refer to as electrolytes, compounds that release ions in water. Electrolytes include salts such as sodium chloride and also acids and bases. In our bodies electrolytes exist primarily in solution with water. The ions released from the electrolytes can be used by our cells. We just formed the compound sodium chloride. Once we eat this salt, in our body fluids, such as inside the digestive tract, much of this compound will separate, releasing sodium ions and chloride ions, both with important roles. Sodium ions, for example, are essential for many body functions, including muscle contraction; conduction of nerve impulses, or nerve signals; and regulation of blood volume and blood pressure. In fact sodium's role in blood pressure is why patients with hypertension, or high blood pressure, are advised to follow a low-sodium diet and a major reason why sodium content is listed on our nutrition labels.