Ions and Their Names

We have discussed earlier how the position of an element on the periodic table determines how likely it is that the atoms of that element will gain or lose electrons.

The position on the periodic table also influences how many electrons are gained or lost, but with a twist. As it turns out, the elements that are most likely to lose electrons do not lose the most electrons. They lose only one electron. Also, the elements with the strongest tendency to gain electrons do not gain the greatest number of electrons. They gain only one electron.

So let's look at how many electrons will be gained or lost by different kinds of atoms. Quite often this can be determined by looking at the position of the element on the periodic table. We will look first at cations, then anions, and then polyatomic ions which are charged clusters of atoms. We'll also look at the names of the ions.

You should get your periodic table out while going through this section (and really almost any work you do in this class from here on).

Cations

First let's look at the metals, which are more likely to lose electrons than gain them. You have learned that when an atom loses an electron or electrons, it becomes positively charged (+) and is called a cation. Thus we can say that in ionic bonding, metals form cations.

Here's an important rule of ion formation: when possible, atoms "like" to form ions with eight valence electrons. This is called the octet rule, and we will revisit it in Lesson 8. Another way of stating the octet rule for ions is that ions like to have the same electron configurations as noble gases. Let's see this in action.

Predictable Cations

Sodium has an electron configuration of 1s² 2s² 2p⁶ 3s¹. You should be able to determine this with your periodic table. The noble gas neon has a configuration of 1s² 2s² 2p⁶. Thus, the optimal route for sodium is to lose exactly 1 electron, giving it a +1 charge. And this is indeed what we observe: sodium only ever forms Na⁺ ions in nature. Not only is this true for sodium, it is true for all the group IA metals: Li, Na, K, Rb, Cs, and Fr.

Predictable Cation Names

It's important we be able to refer to things with names. When naming ions from these three predictable groups, the method is simple: just use the name of the element. That is, if you are talking about ions, "magnesium" means Mg²⁺, "aluminum" means Al³⁺, etc. So, if I say "write the symbol for the rubidium ion" you should be able to correctly write Rb⁺, and if I ask for the name of Ga³⁺, you should be able to say it is the gallium ion.

Unpredictable Cations

As the image above indicates, things get murkier with the transition metals. First, a given transition metal element will often form multiple ions, depending on the conditions of the reaction. For example, iron commonly forms both Fe²⁺ and Fe³⁺ ions. The term for elements that behave this way is multivalent.

Furthermore, just because elements are in the same column doesn't mean they form ions with the same charge. Copper forms both +1 and +2 ions; silver forms exclusively +1 ions; and gold forms both +1 and +3 ions.

You will not be required to memorize or predict the charges adopted by transition metal ions. However, you will be expected to work with them when determining ionic compound names and formulas (which will be laid out in the coming sections).

Unpredictable Cation Names

Naming these ions will not be quite as simple as it was for the predictable ions. This is because of the need for unambiguous communication. In the case of Mg, I can say "the magnesium ion" you know unambiguously that I am referring to Mg²⁺, because there is a rule for the charge adopted by that element. In the case of iron, if I say "the iron ion," you don't know whether I mean Fe²⁺ or Fe³⁺. This is a problem.

We resolve the issue by adding a roman numeral in parentheses at the end of transition metal names. So the Fe²⁺ ion is denoted "iron (II)" and the Fe³⁺ ion is denoted "iron (III)." It's that simple. Take a moment and practice writing names for the following ions: V³⁺, Rh⁴⁺, Au⁺, Cu²⁺

One of the places students commonly go wrong in the next few sections is they either forget to use roman numerals or they forget their meaning. It's very important you get used to always using a roman numeral when naming anything involving a transition metal. And it's equally important that you not forget what the roman numeral indicates: the positive charge on the ion.

*Note: an older naming system exists for multivalent cations based not on roman numerals but on suffixes. For example, the iron ions above would be called ferric (Fe³⁺) and ferrous (Fe²⁺). This system is antiquated. As you can see here, it sometimes uses Latin roots instead of the English names of the elements. Also, in my opinion, it is far less accessible to introductory students because it requires you to memorize, for example, that the "cupric" ion has a +2 charge and the "cuprous" ion a +1. It will not be used in this class, and you should avoid answering questions with it. I am telling you about it because it is not uncommon to run into it if you are doing research online or elsewhere.

Anions

Next let's deal with the nonmetals. You have learned that when an atom gains an electron or electrons, it becomes negatively charged (-) and is called an anion. Thus we can say that in ionic bonding, non-metals form anions.

*Note: as indicated in the previous section, non-metals can also bond with other non-metals or with themselves in covalent bonding, addressed in Lesson 8. When this occurs, the atoms are not becoming anions. I mention this so you will not internalize the idea "non-metals always form anions," as it is incorrect. Non-metals form anions when they engage in ionic bonding.

Anion Charges

Fortunately, anions are much simpler than cations. None of the non-metals are multivalent, so they all form predictable charges, which can be worked out with the same logic we used above for metals.

Anion Names

The naming of anions is also quite simple. To name an ion like Se^2-, we mostly keep the name of the element; we just change the ending to "-ide." So selenium (Se) becomes selenide (Se^2-). Following this pattern, we can list the names of all the non-metal anions here: nitride, phosphide, oxide, sulfide, selenide, fluoride, chloride, bromide, iodide, and hydride (the H^- ion, which is rather rare but can be formed).

Polyatomic Ions

It is also possible for several nonmetal atoms to cluster together in groups and form ions. These are called polyatomic ions. The formulas and charges of these polyatomic ions are not nearly so predictable as the simple anions you just studied. Nor are they all anions. The eight that you will need to memorize are listed here.

You need to know the names and formulas (including charges) for these eight polyatomic ions. Unfortunately, there is no "easy" way to predict their formulas/charges from the periodic table; you simply have to memorize them. Making flashcards and quizzing yourself throughout the week is one technique to learn them before the quiz.

You may find it helpful in memorizing these ions to use a mnemonic like this one. It is a bit silly, but it gives you a memorable and systematic tool for remembering the charges and number of oxygens on carbonate, nitrate, sulfate, phosphate, and chlorate.