Ionic Compound Names and Formulas

It is possible to figure out or predict the formulas of ionic compounds just from knowing which elements are involved by considering the charges on the ions.

In the following sections are several examples of how to determine formulas and names of ionic compounds. Each example tries to model the step-by-step thought process you can use to figure out ionic formulas and names. In sequence, each presents an additional factor or level of complexity. It will be handy to have a periodic table with you as you go through this.

Principles To Follow

In the following examples, here are the principles we will need to keep in mind.

• In naming and writing formulas for ionic compounds, the cation will always come first, followed by the anion

• The name of an ionic compound consists of the name of the cation followed by the name of the anion, with a space in between

• The formula of an ionic compound is determined by the charges on the ions; the total charges on the anions must balance out the total charges on the cations

• Parentheses will be used in the case of polyatomic ions, if there is more than one of that ion in the formula

• Ionic formulas are ratios (empirical formulas) and therefore must always be in lowest form

With this as our starting point, let's dive in

First Example: Sodium and Chlorine

Sodium is in the first column of the periodic table, so it loses one electron and becomes a sodium ion with a +1 charge.

Chlorine is a nonmetal in the next to the last column. Because it is a nonmetal, it will tend to gain electrons, and it will gain as many electrons as it needs to fill up its outermost energy level. There's room for only one more electron so it will take on a -1 charge and is called a chloride ion.

So we have a +1 charge for sodium and a -1 charge for chloride. The charges are opposite but equal; therefore, there is a 1:1 ratio of sodium ions to chloride ions and NaCl is the formula of the compound.

The name of this ionic compound is sodium chloride because it is made of sodium ions and chloride ions.

Second Example: Magnesium and Bromine

Magnesium is in Group IIA and so it will have a +2 charge when it forms an ion. The ion is called a magnesium ion.

Bromine, being in Group VIIA, has a -1 charge on its bromide ion.

With a +2 charge and a -1 charge, we will need two of the bromide ions to match the amount of charge on one magnesium ion. The formula for this compound is MgBr₂. Note that the charge ratio is two-to-one, so the ion (or atom) ratio is one-to-two.

The name of this ionic compound is magnesium bromid because it is made of magnesium ions and bromide ions.

Third Example: Iron and Chlorine

Iron is a transition metal. With transition metals you cannot count on knowing the ionic charge just from its position in the periodic table. For most transition metals, you will have to look up what charges actually exist for that element. You can look these up in your textbook or you can look back at the list in your workbook. However, you should memorize the charges for copper and iron. Iron has an ion with a +2 charge called iron (II), and one with a +3 charge called iron(III). The chloride ion, of course, has a -1 charge.

Because there are two types of cations, two different compounds can be formed. When the Fe²⁺ combines with chloride, you will get FeCl₂. FeCl₂ is named iron (II) chloride because it is made of iron (II) ions and chloride ions.

If the Fe³⁺ ion combines with chloride, then that will give you FeCl₃ because three chloride ions are needed to balance the charge on a +3 ion. FeCl₃ is named iron(III) chloride because it is made of iron(III) ions and chloride ions.

Fourth Example: Magnesium and Oxygen

Magnesium is a Group IIA element; therefore, it will have a +2 charge on the magnesium ion.

Oxygen is in Group VIA. It has six electrons in its outer shell. It has room for two more so when it forms an ion, it has a -2 charge and it is called an oxide ion.

Here again, we have equal charges on the two ions: a +2 on the magnesium and a -2 charge on the oxygen. So they will combine in a 1:1 ratio and the formula for that compound is MgO.

It is called magnesium oxide because it is made of magnesium ions and oxide ions.

Fifth Example: Aluminum and Oxygen

Aluminum is in Group III-A. It is a metal. Metals lose electrons. It has three electrons in the outermost energy level; therefore, it loses three electrons and forms a +3 ion called an aluminum ion.

Oxygen, of course, takes on a -2 charge to form an oxide ion.

The charge ratio is 3:2 therefore the atom ratio will be 2:3. Since two aluminum ions each with +3 charge will balance the charge on three oxide ions with -2 charge, the formula will be Al₂O₃.

The name is aluminum oxide because it is made from aluminum ions and oxide ions.

Sixth Example: Lithium and Phosphate

Lithium is a Group IA metal, so its ion will have a +1 charge.

Phosphate is a polyatomic ion with a -3 charge.

The compound needs three lithium ions to balance the -3 charge of the phosphate, so the formula is Li₃PO₄. Notice that we didn't do anything with the "4" subscript on the phosphate. That is part of the ion formula and doesn't have anything to do with determining the charge ratios.

The name of the compound is lithium phosphate.

Seventh Example: Magnesium and Acetate

Magnesium is a Group IIA metal, so its ion will have a +2 charge.

Acetate is a polyatomic ion with a -1 charge.

The compound needs two acetate ions to balance the +2 charge on the magnesium. Therefore, the formula is Mg(C₂H₃O₂)₂. We need parentheses around the acetate ion to show that we need two of that whole polyatomic ion - without changing the formula of the acetate.

The name of the compound is magnesium acetate.

Please notice that when we wrote the formula for lithium phosphate, we did not use parentheses. We did not need more than one phosphate ion, so we did not use them. Only use parentheses when you need to use a subscript for your polyatomic ion in the formula, like we did with the magnesium acetate.

You should now go immerse yourself in the practice problems in your lab workbook related to ionic naming and formulas. Seriously, go! This is a skill that requires practice to get good at!