Previously, we’d discussed NaCl, or sodium chloride, several times. This particular substance has been a great studying point, because there is much to learn in chemistry that it can serve as an example for. The newest of those concepts for which we’ll apply our knowledge to an example, using NaCl, is ionic substances. An ion is obtained from an atom or an entire group of atoms that are chemically bonded to one another, by adding to removing electrons, which makes this type of particle electrically charged. Sodium chloride consists of ions (Ebbing and Gammon 2009).
When an atom picks up an extra electron, because an electron is a negatively-charged particle, that atom becomes a negatively charged ion, which is given the name anion. When an atom of chlorine gains an electron, the anion Cl— is formed. When an atom loses an electron (a negatively charged particle), it becomes a positively-charged ion, known as a cation. When an atom of sodium loses an electron, a cation is formed, which is written as Na+. Calcium has the ability to lose two electrons, to form a calcium cation written as Ca2+. It is written this way because the calcium has a positive charge of two (EbbingE and Gammon 2009).
In some cases, two or more atoms bond chemically and there is a normal excess or deficiency of electrons. This makes the entire collection of atoms collectively have a positive or negative charge in the case of deficiency or excess, respectfully. An example of this is the sulfate ion, SO42—. From what you learned so far, you should be able to say confidently that the charge of negative two is due to an excess of two electrons (Ebbing and Gammon 2009). At first, this entire concept may seem confusing to you, as it did to me. It might trip you up to think that when an atom loses something, it can become positively-charged, or that when an atom gains something, it can become negatively-charged. It is easy to make a mistake here because the something in question is an electron, which is negatively charged. If you are like me and sometimes get momentarily confused about what an anion is and what a cation is, just take it slowly.
Ionic compounds are those compounds that are composed of cations and anions. Positive and negative charges on atoms hold ions together due a very strong attraction. Ions are held together in a regular and orderly arrangement in space. Going back to our classic example, in NaCl, each sodium cation is actually surrounded by six chloride anions; likewise, each chloride anion is surrounded by six sodium cations. This forms sort of a crystalline structure, which is a solid, and has a definitive arrangement in 3D. The size of the crystal varies with the number of ions in the sodium chloride crystal; it can be very large or very small (Ebbing and Gammon 2009).
To write the formula for an ionic compound, the smallest integer number of the different ions in the substance is what is used. The charges on ions are left out. For instance, in sodium chloride, there are an equal number of sodium cations as there are chloride ions. The formula is simply NaCl. If you think about it, this makes perfect sense because the negative and positive charges actually form a neutral substance overall. Not all ionic substances have equal numbers of each atom. Another example of an ionic compound is iron(III) sulfate, which consists of iron(III) ions that actually have a positive charge of three, and sulfate ions, SO42—, that obviously have a negative charge of two. The formula for this compound is written as Fe2(SO4)3. The parentheses, in this case, separate an ion that is composed of more than one type of atom from the ions that are made of just one type of atom. And the reason for the subscript of two on the iron ion and the subscript of three on the sulfate ion is because two iron atoms will have a charge of positive six, which is equal to zero, with the negative charge of six on the three sulfate ions (Ebbing and Gammon 2009).
The example we just discussed, namely Fe2(SO4)3, is also an example of something else. Fe2(SO4)3 is also what is known as a formula unit, which signifies the smallest unit of a substance consisting of a group of atoms of ions that are explicitly symbolized in the formula. To reinforce this point, the ions would not be the same if there was just a single iron(III) ion and two sulfate ions. In that case, the charges would not balance and the substance formed would not be neutral overall. You could also come to this conclusion by realizing the fact that the ratio of two iron(III) ions to three sulfate ions is not a reducible ratio. There is no way to reduce this to a smaller integer number of each ion. Therefore, Fe2(SO4)3 is a formula unit definitively (Ebbing and Gammon 2009).
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