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4.4: Polyatomic Ions

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    280727
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    Polyatomic Ions

    Polyatomic Ions are charged molecules, where the total number of protons in all bonded atoms is \(\neq\) to the total number of electrons in all bonded electrons.  When writing structural formulas for ions we place the formula in brackets and the charge as a superscript outside of the bracket.  The number of protons is the total number of protons in all atoms, while the number of protons minus the number of electrons is the charge of the atom.  That is the charge of the ion is the charge of all protons [+ value] plus the charge of all electrons [- value].

    • Ammonium is a polyatomic cation, NH4+.
    clipboard_eb685c68640160d86a7f7e6d8138db236.png
    Figure \(\PageIndex{1}\): Structural formula for ammonia, NH4+.  Note how the formula is placed in a bracket  with the charge outside of the bracket.  You must always indicate the charge of the ion when writing it (Copyright; author via source)

    Exercise \(\PageIndex{1}\)

    How many protons and electrons are in ammonia

    Answer

    There are 7 protons from the nitrogen and 4 from the four hydrogen (1 each) and so a total of 11 protons are in this polyatomic ion, which as drawn, has 4 covalent bonds.  Since the charge is [+1],  \[\#protons -\#electrons= charge \\ \#electrons=\#protons- charge\\ \#electrons=11-1=10 \nonumber \]

     

    • Sulfate is a polyatomic anion, SO4-2.
    clipboard_e12d0427bd3dbb9dff988a9bc4b6b232d.png
    Figure \(\PageIndex{2}\): Structural formula for sulfate, SO4-2.  Note how the formula is placed in a bracket  with the charge outside of the bracket.  You must always indicate the charge of the ion when writing it.

    Exercise \(\PageIndex{1}\)

    How many protons and electrons are in sulfate?

    Answer

    There are 16 protons from the sulfur and 32 from the four oxygen (8 each) and so a total of 48 protons are in this polyatomic ion, which as drawn, has 6 covalent bonds (note two are double bonds).  Since the charge is [-2], \[\#protons -\#electrons= charge \\ \#electrons=\#protons- charge\\ \#electrons=48-(-2)=50 \nonumber\]


    Formula of Polyatomic Ions

    The rules for formula of polyatoic ions are the same as for monatomic ions.  The formula of the neutral compound between ammonium and sulfate is (NH4)2SO4.

    clipboard_e9dce15dcb10eb4cf187c36612eb7a510.png
    Figure \(\PageIndex{3}\): There are two ammonium ions for every sulfate ion.

     

     

     


    4.4: Polyatomic Ions is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

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