Learn how to name positive ions (cations), negative ions (anions), and ionic compounds involving main group elements.
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 | ||||
sodium ion |
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 | ||||
chloride ion |
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.
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
Element | Ion symbol | Ion name |
---|---|---|
hydrogen | hydrogen ion | |
lithium | lithium ion | |
sodium | sodium ion | |
potassium | potassium ion |
In speech, we may also refer to a hydrogen cation simply as "
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 | magnesium ion | |
calcium | calcium ion | |
strontium | strontium ion | |
aluminum | aluminum ion | |
silver | 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
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,
In speech, we have to call
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 | ||
chlorine | chloride | ||
bromine | bromide | ||
iodine | iodide | ||
oxygen | oxide | ||
sulfur | sulfide | ||
nitrogen | nitride | ||
phosphorus | phosphide | ||
carbon | carbide |
Let's use the oxygen anion as an example. In writing, it is shown as the symbol
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
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.
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
, not or some other multiple of , 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 (
(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
Magnesium (
Phosphorus (
Therefore, the name for the ionic compound
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 (
Element | Common ions formed | Systematic name | Common name |
---|---|---|---|
chromium | chromium(II) | chromous | |
chromium(III) | chromic | ||
cobalt | cobalt(II) | ||
cobalt(III) | |||
copper | copper(I) | cuprous | |
copper(II) | cupric | ||
iron | iron(II) | ferrous | |
iron(III) | ferric | ||
lead | lead(II) | ||
lead(IV) | |||
tin | tin(II) | stannous | |
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 | scandium | |
zinc | zinc | |
silver | silver | |
cadmium | 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
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
In order for the compound to be electrically neutral, the lead cation must be
Therefore, the name of
Try it: Names and formulas of ionic compounds
Problem 1
What is the name of the compound
The name of the cation appears first in the names of ionic compounds. So, we know this compound's name will start with strontium.
The name of the anion appears second in the names of ionic compounds. So, we know this compound's name will end with fluoride.
Therefore, the name of the ionic compound
Problem 2
What is the chemical formula for calcium bromide?
Bromide is the name of the bromine anion. From bromine's position in group 17 of the periodic table, we know that the bromine anion has a
Calcium is a group 2 element. Based on calcium's position on the periodic table, we predict it will form a
Next, we need to determine the ratio of
We need one
The ratio of
So, the chemical formula for calcium bromide is
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.