Naming Simple Ionic Compounds

Whenever a chemical formula contains a metal and a nonmetal, the result is an ionic compound, and just as the name indicates, this chemical species is composed of ions. These compounds are very different structurally from covalent compounds. But before continuing, a brief introduction to ions and their nomenclature is in order. In an atom, the number of protons, which have positive charges, equals the number of electrons, which have negative charges. All positive and negative charges cancel out. Ions are atoms that have either lost or gained one or more electrons. The sodium atom, Na, loses an electron to form the sodium ion (Na⁺), and the atom calcium, Ca, loses 2 electrons to form the calcium ion (Ca²⁺). A cation is a positively charged ion. Notice the charge is positive and the number superscript corresponds to the number of electrons lost. Some atoms gain one or more electrons to become negatively charged ions called anions. For example, the fluorine atom, F, gains one electron to form the fluoride ion (F^–), and the oxygen atom gains two electrons to form the oxide ion (O^2-). When atoms gain electrons the charge is negative and the superscript number corresponds to the number of electrons gained. If only one electron is lost or gained, the number “1” is assumed.

Atoms such as sodium and fluorine will form just one ion, Na⁺ and F⁻, whereas other atoms can form more than one ion. For example, iron can lose 2 electrons to form Fe²⁺ or 3 electrons to form Fe³⁺ ions. These simple ions, which are derived from single atoms, are called monatomic ions. The rules for naming monatomic ions are straightforward.

Ions with negative charges are named by:

The element name (drop ending) + add –ide + ion

Some examples are: Cl^– is the chloride ion and S^2- is the sulfide ion

Metals that form positively charged ions of only one charge are named as:

The element name + ion

Some examples are: Na⁺ is the sodium ion and Ca²⁺ is the calcium ion.

Metals that form cations of more than one possible charge are named as:

The element name + roman numeral which indicates the + charge ion.

Fe²⁺ is the iron(II) ion, and Fe³⁺ is the iron(III) ion. These are pronounced as the iron two ion and iron three ion. Ions that can form two or more different cations are also referred to by their common names. For example the common name for the iron(II) ion is the ferrous ion and for iron(III) is the ferric ion. The suffixes –ic and –ous are added to the Latin names of the elements. The suffix –ic is used for the cation with the smaller charge whereas –ous is used for the cation with the higher charge. The table below lists both the systematic and common names for some common metals that can form more than one cation. In this study guide we will use the systematic method for naming these ions. The following table lists some common and systematic names for 4 metals that can form more than one cation.

Since the sodium atom only forms one ion, Na+, it is not necessary to indicate the sodium ion as sodium(I) ion. The same holds for the calcium and potassium ions. The method in determining the charge on these simple monatomic ions employs use of the Periodic Table. Group 1A atoms form ions with a +1 charge, Group 2A atoms form ions with a +2 charge, Aluminum in Group 3A forms an ion with a +3 charge, Group 6A atoms form ions with a -2 charge and Group 7A atoms form ions with a -1 charge. In Group 5A, nitrogen and phosphorus can both gain three electrons to form ions with a -3 charge. The portion of the periodic table below shows the common monotomic ions. You will need to know these.

Full Periodic Table With Charges

The following periodic table shows charges for both monovalent (forms only one charge) and polyvalent (forms more than one charge) metal ions. In my course, you will need to know all of the charges for the metals that are shaded in green. When naming these ions or naming compounds that contain these ions, Roman numerals are not used. Assume that when naming other metal ions (not shaded), Roman numerals are used.

Click the periodic table for a full size version.

Rules for naming binary ionic compounds

1. Identify the cation and anion
2. The cation will have the same name as the element
3. For the anion, modify the ending of the element name, and add the suffix “ide”
4. When naming, the cation name comes first

The formula NaCl consists of a sodium ion (Na⁺) with a charge of +1 and a chloride ion (^–) with a charge of -1. The name of the compound is sodium chloride. Note, the 1 is not included when writing the charge.

The formula MgF₂ consists of one magnesium ion (Mg⁺²) with a charge of +2 and a Fluoride ion (F^–) with a charge of -1. The name of the compound is magnesium fluoride. Notice, there are 2 fluoride ions for each magnesium ion. Compounds must be neutral, therefore the charges must add up to zero for a neutral compound. For MgF₂, two fluroide ions with a -1 charge are required to combine with one Mg⁺² ion to form the neutral MgF₂.

We see Fe₂O₃ is composed of iron and oxygen. Iron is a transition metal that can form more than one charge. In this case we must use Roman numerals to indicate the charge on the iron. First, we must determine the charge on iron. We know the oxide ion (O^2- has a -2 charge. We see the subscript on Fe is 2. This corresponds to the charge on the oxide ion. The subscript on O is 3. The Fe ion has a +3 charge. We must check this to make sure the charges add to zero. In one formula unit, we have 2 Fe and 3 O. Let’s see if they add to zero: 2 x +3 + 3 x -2 = 0. The charges add to zero, therefore, we can write the formula as Fe₂O₃. The name of the compound is iron(III) oxide.

Now, let’s look at MnO₂. The name of this compound is not manganese (II) oxide. At closer inspection, we see 2 oxide ions in the formula. There is a 1:2 ratio of manganese ions to oxide ions. Each oxide ion has a -2 charge for a total of 2 x -2 = -4. For MnO₂ to be neutral, the manganese ion must have a charge of +4, Mn⁺⁴. The name of this compound is manganese(IV) oxide. So, why not write the formula as Mn₂O₄? The formula of an ionic compound must be written to show the smallest whole number ratio (empirical formula) between the cations and anions. The number of ions in a sample of an ionic salt will be very large and and of course, will vary between samples. For covalent compounds, the smallest unit is the molecule, therefore, we can write the exact number of atoms in one molecule of the compound since the compound contains discrete molecules. For ionic compounds, we can only represent the number of ions in one formula unit–the simplest whole number ratio of cations:anions–because ionic compounds are not made up of discrete molecules. We will talk more about empirical formulas in a later section.

Please note that compounds that contain ammonium ion, NH₄⁺ are also ionic compounds. For example, NH₄Cl is named ammonium chloride, and (NH₄)₂S is named ammonium sulfide.

Worksheet: Nomenclature
Worksheet: Ions, Cations, and Anions