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Course: High school chemistry > Unit 2
Lesson 1: Ionic bondsNaming ions and ionic compounds
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 charge. The following table shows how we name the ions for the first four elements in this group:
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 " -plus". Similarly, a sodium cation can be called "a sodium ion" or " -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 | 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.
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 or written as "oxide". In speech, we'd typically refer to it as an "oxygen ion", "oxide", or " -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 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.
- 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 ( ) is in group 1, so it forms a cation with a charge. Chloride is, by definition, an anion formed from an atom of chlorine ( ) . Chlorine is in group 17, so chloride has a charge. The potassium and chloride ions have equal and opposite charges, so they will join in a 1:1 ratio, forming .
(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 ( ) is in group 2, so it forms a cation with a 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 ( ) is in group 15, so it forms anions with a 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 is magnesium phosphide.
Try it: Names and formulas of ionic compounds
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.
Want to join the conversation?
- We passing chemistry class with this one !(22 votes)
- How do you name cations? If anions have their endings replaced with, "-ide", is there a different suffix for cations?(5 votes)
- Naming cations typically involves using the name of the element followed by the word "ion" or simply the name of the element itself. Unlike anions, cations do not have a specific suffix like "-ide." They are named based on the name of the element from which they are derived. For example:
Sodium cation: Na⁺
Calcium cation: Ca²⁺
Aluminum cation: Al³⁺
So, for cations, the naming convention is simpler compared to anions, and no suffix like "-ide" is used.-hope this helps!(4 votes)