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3.5.1: Ions - Monatomic and Polyatomic

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    • Anonymous
    • LibreTexts
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    Learning Objectives
    • Determine the charges of monatomic and polyatomic ions.

    Correctly determining the formulas of ionic compounds requires an understanding of how ions form and what charges they have. In a previous section we discussed how anions and cations might form from gaining or losing electrons, respectively. In this section we will discuss some common charges of ions that you will need to know in order to predict the correct formulas of ionic compounds. We will look at both monatomic and polyatomic ions. In a later chapter, we will learn why the monatomic ions form the charges that they do. The polyatomic ions, however, are much more difficult to predict.

    Monatomic Ions

    In many cases, elements that belong to the same group (vertical column) on the periodic table form ions with the same charge. Thus, the periodic table becomes a tool for remembering the charges on many ions. For example, all ions made from alkali metals, the first column on the periodic table, have a 1+ charge. Ions made from alkaline earth metals, the second group on the periodic table, have a 2+ charge. On the other side of the periodic table, the next-to-last column, the halogens, form ions having a 1− charge. Figure \(\PageIndex{3}\) shows how the charge on many ions can be predicted by the location of an element on the periodic table. Note the convention of first writing the number and then the sign on a ion with multiple charges. The barium cation is written Ba2+, not Ba+2.

    Figure \(\PageIndex{3}\): Predicting Ionic Charges. The charge that an atom acquires when it becomes an ion is related to the structure of the periodic table. Within a group (family) of elements, atoms form ions of a certain charge.

    Polyatomic Ions

    Some ions consist of groups of atoms bonded together and have an overall electric charge. Because these ions contain more than one atom, they are called polyatomic ions. Polyatomic ions have characteristic formulas, names, and charges that should be memorized. For example, NO3 is the nitrate ion; it has one nitrogen atom and three oxygen atoms and an overall 1− charge. Table \(\PageIndex{1}\) lists the most common polyatomic ions.

    Table \(\PageIndex{1}\): Some Polyatomic Ions
    Name Formula
    ammonium ion NH4+
    acetate ion C2H3O2 (also written CH3CO2)
    carbonate ion CO32
    chromate ion CrO42
    dichromate ion Cr2O72
    hydrogen carbonate ion (bicarbonate ion) HCO3
    cyanide ion CN
    hydroxide ion OH
    nitrate ion NO3
    nitrite ion NO2
    permanganate ion MnO4
    phosphate ion PO43
    hydrogen phosphate ion HPO42
    dihydrogen phosphate ion H2PO4
    sulfate ion SO42
    hydrogen sulfate ion (bisulfate ion) HSO4
    sulfite ion SO32

    Please note that many of the polyatomic ions have names which are only different from each other based on the last 3 letters. Also, that there will often be monatomic ions which also differ from polyatomic ions only in the last 3 letters of the name. For example, nitrate, nitrite, and nitride are each different ions. The formulas for nitrate and nitrite are listed in the table below, and you can see that they have different numbers of oxygen within their formulas. Nitride is a monatomic ion, so you would consult the periodic table to determine its formula and charge. Note that any time in ion hasn’t ending of -ate or -ite it is a polyatomic ion. Ions which ended with -ide could be either monatomic or polyatomic. One of the most important parts of naming ionic compounds correctly is paying attention to those last 3 letters.

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