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4.5: Naming Ionic Compounds

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    49807
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    Skills to Develop

    • Correctly name binary ionic compounds, compounds containing metals with variable oxidation numbers, and compounds containing polyatomic ions given the formulas.

    We have already learned about naming individual ions, including main group ions, transition metal ions, and polyatomic ions. We have also learned how to put these into correct charge-balanced formulas. In this section, we will learn how to correctly name a compound, given its formula.

    Naming Ionic Compounds

    To name ionic compounds, we will need to follow these steps:

    1. Split the formula into the cation and anion. The first metal listed will be the cation and the remaining element(s) will form the anion.

    2. Name the cation. We learned two types of cations:

    a. Main group cations in which the name of the ion is the same as that of the element (for example, \(\ce{K^+}\) is potassium).

    b. Transition metals with variable charges with Roman numerals indicating the charge of the ion (you will have to do a little bit of math to find this charge).

    3. Name the anion. There are also two general types of anions:

    a. Main group anions in which the name of the anion ends in "-ide" (for example, \(\ce{F^-}\) is fluoride)

    b. Polyatomic ions (as listed on the polyatomic ion chart)

    When writing the name of an ionic compound, it is important to note that the name gives no information about the number of ions. He name only tells the types of ions present. The formula uses subscripts to indicate how many of each ion there are.

    Example 4.5.1

    What is the name of \(\ce{Na_2O}\)?

    Solution

    Split up the formula: \(\ce{Na} \: | \: \ce{O}\)

    Name the cation: \(\ce{Na}\) is a group 1 metal with a consistent charge. It does not need Roman numerals. Its name is "sodium"

    Name the anion: \(\ce{O}\) is not polyatomic. When oxygen atoms get a \(-2\) charge, the name changes to end in -ide, so the anion is "oxide"

    Final answer: sodium oxide

    Example 4.5.2

    What is the name of \(\ce{NaC_2H_3O_2}\)?

    Solution:

    Split up the formula: \(\ce{Na} \: | \: \ce{C_2H_3O_2}\)

    Name the cation: \(\ce{Na}\) is a group 1 metal with a consistent charge. It does not need Roman numerals. Its name is "sodium"

    Name the anion: \(\ce{C_2H_3O_2}\) is polyatomic. Its name is "acetate"

    Final answer: sodium acetate

    Example 4.5.3

    Write the name of \(\ce{CuCl_2}\).

    Solution:

    Split up the formula: \(\ce{Cu} \: | \: \ce{Cl_2}\)

    Name the cation: \(\ce{Cu}\) is a transition metal with a variable charge. It needs Roman numerals. To find the charge, consider the charge of the other ion and the number of both ions: \(\ce{Cu_1^{?}} \ce{Cl_2^{-1}}\). The copper must have a charge of \(+2\) to balance the negatives: \(1 \cdot \left( +2 \right)\) to cancel out \(2 \cdot \left( -1 \right)\). Its name is "copper (II)"

    Name the anion: \(\ce{Cl}\) is not polyatomic. When chlorine atoms get a \(-1\) charge, the name changes to end in -ide, so the anion is "chloride"

    Final answer: copper (II) chloride

    Example 4.5.4

    Write the name of \(\ce{PbS_2}\).

    Solution:

    Split up the formula: \(\ce{Pb} \: | \: \ce{S_2}\)

    Name the cation: \(\ce{Pb}\) is a post-transition metal with a variable charge. It needs Roman numerals. To find the charge, consider the charge of the other ion and the number of both ions: \(\ce{Pb_1^{?}} \ce{S_2^{-2}}\). The lead must have a charge of \(+4\) to balance out the negatives: \(1 \cdot \left( +4 \right)\) to cancel out \(2 \cdot \left( -2 \right)\). Its name is "lead (IV)"

    Name the anion: \(\ce{S}\) is not polyatomic. When sulfur atoms get a \(-2\) charge, the name changes to end in -ide, so the anion is "sulfide"

    Final answer: Lead (IV) sulfide

    he most common error made by students in naming these compounds is to choose the Roman numeral based on the number of atoms of the metal instead of the charge of the metal. For example, in \(\ce{PbS}\), the oxidation state of lead \(\ce{Pb}\) is \(+4\) so the Roman numeral following the name lead is "IV." Notice that there is no four in the formula. As in previous examples, the formula is always the lowest whole number ratio of the ions involved. Think carefully when you encounter variable charge metals. Make note that the Roman numeral does not appear in the formula but does appear in the name.

    Example 4.5.5

    Write the name of \(\ce{Mg(PO_4)_2}\).

    Solution

    Split up the formula: \(\ce{Mg} \: | \: \ce{(PO_4)_2}\)

    Name the cation: \(\ce{Mg}\) is a group 2 metal with a consistent charge. It does not need Roman numerals. Its name is "magnesium"

    Name the anion: \(\ce{PO_4^{3-}}\) is polyatomic. Its name is "phosphate"

    Final answer: magnesium phosphate

    Example 4.5.6

    Write the name of \(\ce{Cr(NO_2)_3}\).

    Solution

    Split up the formula: \(\ce{Cr} \: | \: \ce{(NO_2)_3}\)

    Name the cation: \(\ce{Cr}\) is a transition metal with a variable charge. It needs Roman numerals. To find the charge, consider the charge of the other ion and the number of both ions: \(\ce{Cr_1^{?}} \ce{(NO_2)_3^{-1}}\). The chromium must have a charge of \(+3\) to balance out the negatives: \(1 \cdot \left( +3 \right)\) to cancel out \(3 \cdot \left( -1 \right)\). Its name is "chromium (III)"

    Name the anion: \(\ce{NO_2^-}\) is polyatomic. Its name is "nitrite"

    Final answer: chromium (III) nitrite

    Remember, when writing the name of an ionic compound, it is important to note that the name gives no information about the number of ions. The name only tells the types of ions present. The formula uses subscripts to indicate how many of each ion there are.

    Lesson Summary

    • Ionic bonds are formed by transferring electrons from metals to nonmetals after which the oppositely charged ions are attracted to each other.
    • Ionic compounds form crystal lattice structures rather than molecules.
    • Binary ionic compounds are named by naming the metal first followed by the nonmetal with the ending of the nonmetal changed to "-ide."
    • Compounds containing polyatomic ions are named with the name of the polyatomic ion in the place of the metal or nonmetal or both with no changes in the name of the polyatomic ion.
    • Compounds containing variable oxidation number metals are named with Roman numerals in parentheses following the name of the metal and indicating the oxidation number of the metal.

    Vocabulary

    • Anion: An ion with a negative charge.
    • Cation: An ion with a positive charge.
    • Chemical nomenclature: The system for naming chemical compounds.
    • Ionic bond: The electrostatic attraction between ions of opposite charge.
    • Polyatomic ion: A group of atoms bonded to each other covalently but possessing an overall charge.

    Further Reading/Supplementary Links

    Contributors


    4.5: Naming Ionic Compounds is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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