Polyatomic Ions

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How to Memorize and Name Polyatomic Ions

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Key Questions

  • The prefix poly- means many and the prefix mono- means one.

    Therefore, polyatomic ions are composed of many atoms.

    Examples include: #NO_3^(-1)# and #PO_4^(-3)#.

    The nitrate ion #NO_3^(-1)# is composed of one nitrogen atoms and three oxygen atoms arranged with an imbalance of one extra electron.

    The phosphate ion #PO_4^(-3)# is composed of one phosphate ion and four oxygen atoms arranges with an imbalance of three extra electrons.

    Most polyatomic ions are negative with the main exceptions being #NH_4^(+1)# and #H_3O^(+1)#. The ammonium ion which is composed of one nitrogen and 4 hydrogen with an imbalance of one less electron. The hydronium ion is created when water takes on a dissociated #H^+# ion in an acid solution.

    Monatomic ions are single atoms with a positive or negative charge like Calcium #Ca^(+2)# having two less electrons and Chloride #Cl^(-1)# having one more electron.

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  • The same reason monatomic ions have a charge - due to an imbalance between the number of protons and electrons.

    Polyatomic ions are composed of two or more covalently bonded atoms - similar to that of a neutral molecule. However, as the name implies, this group of covalently bonded atoms has a difference in the number of electrons v protons. So in the formation of the cyanide ion, for example, CN-, carbon and nitrogen bond together through the formation of a carbon-nitrogen triple bond.

    However, if we draw the appropriate structure, you will see that in order for the carbon and nitrogen to achieve stable electron configurations, they will need one more electron than the two of them have collectively - so they obtain an additional electron from a metal donor.

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  • Poly atomic ions are covalent compounds that have an overall charge and therefore are held together through the electrostatic attraction of ionic bonding to positively charged ions called cations.

    With the exception of ammonium, #NH_4^+#, these ions carry a negative

    Examples include sulfate, #SO_4^-2#, nitrate, #NO_3^-#, and phosphate, #PO_4^-3#.

    If we consider nitrate it will form one ionic bond with cation , such as sodium that has + one charge. The formula is #NaNO_3#

    If nitrate bonds with calcium, two nitrate ions each with one calcium ion, #Ca(NO_3)_2#

    If nitrate bonds with aluminum which has a plus three charge, three nitrates are required to create an ionic compound, #Al(NO_3)_3#.


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