Writing Ionic Formulas

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Type I Binary Ionic Compounds - Naming and Writing Formulas

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Key Questions

  • What you want to do is make the compound neutral.

    Let's take the following example:

    #Na^(+)# + #SO_4^(2-)#

    We need to balance the charges, the easiest way to balance this charge is by looking at the overall charge of the ions involved. The #Na# ion has a #+1# charge and the #SO_4# ion has a #-2# charge. In order to give balance, we must have two Na ions to give an overall #+2# with regards to Na: this, thus, neutralises the compound. Therefore, the formula is:


    If you're asked to balance an ionic compound such as Iron(III) Hydroxide, write down the formula. We know that Fe (Iron) has a #3+# charge and the hydroxide ion (OH) has a #1-# charge - as a result, the compounds in their individualised forms are:

    #Fe^(3+)# and #OH^(-)#

    In order to balance this, we need to add brackets around the hydroxide ion to give:


    This balances the charges and, thus, an accurate ionic formula has been given.

  • Lets take the ionic formula for Calcium Chloride is #CaCl_2#

    Calcium is an Alkaline Earth Metal in the second column of the periodic table. This means that calcium has 2 valence electrons it readily gives away in order to seek the stability of the octet. This makes calcium a #Ca^(+2)# cation.

    Chlorine is a Halogen in the 17th column or p5 group.
    Chlorine has 7 valence electrons. It needs one electron to make it stable at 8 electrons in its valence shells. This makes chlorine a #Cl^(−1)# anion.

    Ionic bonds form when the charges between the metal cation and non-metal anion are equal and opposite. This means that two #Cl^(−1)# anions will balance with one #Ca^(+2)# cation.

    This makes the formula for calcium chloride, #CaCl_2#.

    For the example Aluminum Oxide #Al_2O_3#

    Aluminum has an oxidation state of +3 or Al+3
    Oxygen has an oxidation state of -2 or #O^−2#

    The common multiple of 2 and 3 is 6.?

    We will need 2 aluminum atoms to get a +6 charge and 3 oxygen atoms to get a -6 charge. When the charges are equal and opposite the atoms will bond as #Al_2O_3#.

    I hope this is helpful.

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