Naming ions and ionic compounds (article)

Learn how to name positive ions (cations), negative ions (anions), and ionic compounds involving main group elements.

Close up view of colorless sodium chloride crystals, which have a cubic shape.

Cations and anions

When a neutral atom loses one or more electrons, the total number of electrons decreases while the number of protons in the nucleus remains the same. The result is that the atom becomes a cation—an ion with a net positive charge. For example, the element sodium loses one electron to become a cation:

Element name	Symbol	Protons	Electrons	Charge
sodium	$Na$ ‍	$11$ ‍	$11$ ‍	$0$ ‍
sodium ion	$Na^{+}$ ‍	$11$ ‍	$10$ ‍	$1 +$ ‍

Similarly, when a neutral atom gains one or more electrons, the result is that the atom becomes an anion—an ion with a net negative charge. For example, the element chlorine gains one electron to become an anion:

Element name	Symbol	Protons	Electrons	Charge
chlorine	$Cl$ ‍	$17$ ‍	$17$ ‍	$0$ ‍
chloride ion	$Cl^{-}$ ‍	$17$ ‍	$18$ ‍	$1 -$ ‍

Main group elements (the elements in groups 1, 2, and 13-18 of the periodic table) typically form ions of only one charge. The diagram below shows the common charge of ions in different groups.

A periodic table showing the common charges of ions formed by elements in different groups. Group 1, the first column, has a column charge of 1+. Group 2 has a common charge of 2+. Group 13 = 3+. Group 14 = 4+ or 4-. Group 15 = 3-. Group 16 = 2-. Group 17 = 1-. Elements in groups 3 through 12 form multiple charges.

Note: Hydrogen is somewhat unusual in that it readily forms both cations and anions. Most elements form only one or the other.

Naming cations

How do we name main group cations? In general, the cation name is the same as the element name, but with the word "ion" added to the end.

For example, the alkali metals in group 1 on the periodic table tend to form cations with a $1 +$ ‍ charge. The following table shows how we name the ions for the first four elements in this group:

Element	Ion symbol	Ion name
hydrogen	$H^{+}$ ‍	hydrogen ion
lithium	$Li^{+}$ ‍	lithium ion
sodium	$Na^{+}$ ‍	sodium ion
potassium	$K^{+}$ ‍	potassium ion

In speech, we may also refer to a hydrogen cation simply as " $H$ ‍-plus". Similarly, a sodium cation can be called "a sodium ion" or " $Na$ ‍-plus".

The same rule applies to all other elements that typically form cations of one particular charge. This includes the alkaline earth metals in group 2 and a few other metals you may be familiar with, shown in the table below:

Element	Ion symbol	Ion name
magnesium	$Mg^{2 +}$ ‍	magnesium ion
calcium	$Ca^{2 +}$ ‍	calcium ion
strontium	$Sr^{2 +}$ ‍	strontium ion
aluminum	$Al^{3 +}$ ‍	aluminum ion
silver	$Ag^{+}$ ‍	silver ion

Why don't we need to include a charge in the names of these ions? For example, why can we say "calcium ion" instead of "calcium-two-plus ion"? Since these main group elements usually form ions of only one charge, which can be predicted from the periodic table, the charge is implied.

So far, we've considered elements that typically form cations of one particular charge. For example, the alkali metals and the alkaline earth metals usually form 1+ ions and 2+ ions, respectively.

Most transition metals and several metals in groups 13-17, however, can form cations of various charges. For instance, iron is often found as both the ${Fe}^{2 +}$ ‍ and ${Fe}^{3 +}$ ‍ cations, and sometimes other charges as well. Thus, iron is polyvalent, which literally means "many valued"—it is able to form cations of different charges.

For metals that are polyvalent, we need to specify the magnitude of the charge on the ion. We do this by adding Roman numerals to the name, which indicate the charge of the cation. For example, ${Fe}^{2 +}$ ‍ is written as "iron(II)", and ${Fe}^{3 +}$ ‍ is written as "iron(III)."

In speech, we have to call ${Fe}^{2 +}$ ‍ "iron two-plus" or "iron two" because simply referring to it as an "iron ion" will not give enough information to specify the cation charge.

Naming anions

To name main group anions, we take the root of the element's name and add -ide to the end. The following table shows how this suffix is used to name anions of various elements:

Element name	Ion name	Ion formula
hydrogen	hydride	$H^{-}$ ‍
chlorine	chloride	${Cl}^{-}$ ‍
bromine	bromide	${Br}^{-}$ ‍
iodine	iodide	$I^{-}$ ‍
oxygen	oxide	$O^{2 -}$ ‍
sulfur	sulfide	$S^{2 -}$ ‍
nitrogen	nitride	$N^{3 -}$ ‍
phosphorus	phosphide	$P^{3 -}$ ‍
carbon	carbide	$C^{4 -}$ ‍

Let's use the oxygen anion as an example. In writing, it is shown as the symbol $O^{2 -}$ ‍ or written as "oxide". In speech, we'd typically refer to it as an "oxygen ion", "oxide", or " $O$ ‍-two-minus"

Since we can predict the charge of main group anions by their location on the periodic table, it's usually not necessary to specify their charge in the name. For example, it is implied that an oxide ion has a $2 -$ ‍ charge. We don't need to say "oxide-two-minus".

Formulas and naming of basic ionic compounds

Now that we've seen the naming conventions for cations and anions, we can discuss how they apply to naming simple ionic compounds (a.k.a. salts). The following guidelines are used for naming ionic compounds:

Always name the cation before the anion. The cation will appear before the anion in the chemical formula, too.
Any ionic compound will have a net charge of zero. Another way of saying this is that cations and anions must always combine in such a way so that their charges cancel.
See Also
3.3: Naming Ionic Compounds Cation vs Anion: Definition, Chart and the Periodic Table 4.4: Naming Ionic Compounds 5.7: Naming Ionic Compounds
The number of cations and anions in the formula should be written as the lowest possible integer value. For example, the formula for sodium chloride is $NaCl$ ‍, not ${Na}_{2} {Cl}_{2}$ ‍ or some other multiple of $NaCl$ ‍, even though the charges would still add up to zero.

Let's look at a few examples:

Example 1: Finding the chemical formula from the name

What is the chemical formula of potassium chloride?

Potassium ( $K$ ‍) is in group 1, so it forms a cation with a $1 +$ ‍ charge. Chloride is, by definition, an anion formed from an atom of chlorine ( $Cl$ ‍) . Chlorine is in group 17, so chloride has a $1 -$ ‍ charge. The potassium and chloride ions have equal and opposite charges, so they will join in a 1:1 ratio, forming $KCl$ ‍.

(Remember that subscripts are not used when there is only one atom/ion of a particular type.)

Example 2: Finding the name from the chemical formula

What is the name of the ionic compound ${Mg}_{3} P_{2}$ ‍?

Magnesium ( $Mg$ ‍) is in group 2, so it forms a cation with a $2 +$ ‍ charge. Because it is the cation in the compound, we put it first in the compound’s name and refer to it as just magnesium. (Notice we remove the word "ion" since it is now part of a compound).

Phosphorus ( $P$ ‍) is in group 15, so it forms anions with a $3 -$ ‍ charge. Because it is the anion in the compound, we put it second in the compound’s name and refer to the anion by its name, phosphide.

Therefore, the name for the ionic compound ${Mg}_{3} P_{2}$ ‍ is magnesium phosphide.

If an element can form more than one type of cation, we have to specify the charge on that cation. The magnitude of the charge for a transition metal cation is usually indicated using Roman numerals in parentheses after the name of the metal—this is also called the systematic name of the ion. The following table lists some of the most common ions for polyvalent metals.

For some ions, the common name is also given. The common names are somewhat old fashioned nowadays, but they're still used in some cases. Notice that the ions of lesser charge take the suffix -ous in the common name; ions of higher charge take the -ic suffix.

For example, ferrous chloride ( ${FeCl}_{2}$ ‍) is the common name of iron(II) chloride and contains the ${Fe}^{2 +}$ ‍ ion. Ferric chloride ( ${FeCl}_{3}$ ‍) is the common name for iron(III) chloride and contains the ${Fe}^{3 +}$ ‍ ion.

Element	Common ions formed	Systematic name	Common name
chromium	${Cr}^{2 +}$ ‍	chromium(II)	chromous
	${Cr}^{3 +}$ ‍	chromium(III)	chromic
cobalt	${Co}^{2 +}$ ‍	cobalt(II)
	${Co}^{3 +}$ ‍	cobalt(III)
copper	${Cu}^{+}$ ‍	copper(I)	cuprous
	${Cu}^{2 +}$ ‍	copper(II)	cupric
iron	${Fe}^{2 +}$ ‍	iron(II)	ferrous
	${Fe}^{3 +}$ ‍	iron(III)	ferric
lead	${Pb}^{2 +}$ ‍	lead(II)
	${Pb}^{4 +}$ ‍	lead(IV)
tin	${Sn}^{2 +}$ ‍	tin(II)	stannous
	${Sn}^{4 +}$ ‍	tin(IV)	stannic

Note: Roman numerals are only needed for polyvalent elements. Elements which form ions of only one charge do not require Roman numerals.

The following transition metals (groups 3-12) do not require a Roman numeral in their name since they only form one cation:

Element	Ion symbol	Systematic name
scandium	$Sc^{3 +}$ ‍	scandium
zinc	$Zn^{2 +}$ ‍	zinc
silver	$Ag^{+}$ ‍	silver
cadmium	$Cd^{2 +}$ ‍	cadmium

Using this table as a reference, let's practice naming an ionic compound containing a polyvalent metal.

What is the name of the compound ${PbCl}_{4}$ ‍?

When naming ionic compounds that contain transition metals, we first need to determine the charge on the transition metal cation. We can deduce this charge by first calculating the charge contributed by the anion, whose charge we already know for sure.

We recognize that $Cl$ ‍ is a group 17 halogen, so it forms the chloride anion ${Cl}^{-}$ ‍. We can see from the chemical formula ${PbCl}_{4}$ ‍ that there are four chloride ions in the compound. The total negative charge contributed by the four chloride ions is calculated below:

$Total charge from anions = 4 \times (1$ ‍ $-$ ‍ $) = 4$ ‍ $-$ ‍

In order for the compound to be electrically neutral, the lead cation must be ${Pb}^{4 +}$ ‍. This is because the $4 +$ ‍ charge on this ion will exactly cancel the net $4 -$ ‍ charge contributed by the four chloride ions.

Therefore, the name of ${PbCl}_{4}$ ‍ is lead(IV) chloride.

Try it: Names and formulas of ionic compounds

Problem 1

What is the name of the compound ${SrF}_{2}$ ‍?

Choose 1 answer:

Conclusion

Cations are positively charged ions formed when neutral atoms lose electrons; anions are negatively charged ions formed when neutral atoms gain electrons. It is possible to predict the charges of main group ions by looking at the group numbers on the periodic table.

To name main group cations, we simply refer to them as the element's name + "ion". To name main group anions, we add the suffix -ide to the end of the element's name.

Cations and anions combine to form ionic compounds. Ionic compounds are named with the cation first and the anion last. The same convention is used when writing their chemical formulas. Ionic compounds must be electrically neutral. Therefore, the cations and anions combine in such a way that the net charge contributed by the total number of cations exactly cancels the net charge contributed by the total number of anions.

Zumdahl, S. S., and S. A. Zumdahl. "Atomic Structure and Periodicity." In Chemistry, 290-94. 6th ed. Boston, MA: Houghton Mifflin Company, 2003.

Naming ions and ionic compounds (article) | Khan Academy (2024)