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4.12 Ion Charge and the Formulas of Ionic Compounds

4.12 Ion Charge and the Formulas of Ionic Compounds. (Sec 8.2 pg. 238). In the last section we looked at ions, and drawing compounds with multiple ions in them. Now we will examine why ions combine in the ratios they do. . Two Element Compounds: These are called 'binary' compounds.

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4.12 Ion Charge and the Formulas of Ionic Compounds

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  1. 4.12 Ion Charge and the Formulas of Ionic Compounds (Sec 8.2 pg. 238)

  2. In the last section we looked at ions, and drawing compounds with multiple ions in them. • Now we will examine why ions combine in the ratios they do.

  3. Two Element Compounds: • These are called 'binary' compounds. • They have a positive ion (Cation) and a negative ion (Anion). • Reminder: we’re only looking at Ionic compounds in this course – they contain a metal and non-metal.

  4. Two Element Ionic Compounds: • The positive ion is listed first and then the negative ion. • The total positive charge has to equal the total negative charge. • Combine Ca2+ with P3- • Ca2+ + Ca2+ + Ca2+ = Ca36+ Combines with P3- + P3- = P26- • Ca3P2 • (Notice that the total negative charge equalled the total positive charge, and once combined we don’t write the charges) • Check out the 'Quick Trick' to Balance Charges pg.240 of your text. • NOTE: Always write the formula in lowest terms. • Combine Al3+ and P3- • Al3P3 is balanced but not in lowest terms = AlP (you only need one of each ion for the compound to have balanced charges).

  5. Ionic compounds with Polyatomic Ions: • These compounds still have a positive and negative ion component. • One group of atoms (polyatomic ions) has the charge instead of a single atom (the whole group of atoms carries the ion charge). • e.g. NH4+ The charge is on the whole group, not just H. • These combine just like binary compounds, except now you use a group instead of a single ion. Often necessary to put brackets () around groups. • Combine Ca2+ with PO43- • Ca2+ + Ca2+ + Ca2+ = Ca36+ , PO43- + PO43- = (PO4)26- • Ca3(PO4)2 • (Notice that the total negative charge equalled the total positive charge, and once combined we don’t write the charges)

  6. Ionic compounds with Multivalent Ions: • These still have a positive and a negative ion component. • If the positive ion can have multiple charges, we call it multivalent (many transition metals are multivalent - see Table 8 pg.242). • A roman numeral shows which ion to use when developing a formula. • e.g. Iron (III) = Fe3+ • In order to make a multivalent compound formula you need to pay attention to the name of the compound. • What is the formula for iron (II) nitride? • iron (II) = Fe2+ and nitride = N3- • Fe2+ + Fe2+ + Fe2+ = Fe36+ , N3- + N3- = N26- • Fe3N2

  7. 4.13 Naming Ionic Compounds (Sec 8.3 pg 245-246)

  8. Naming ionic compounds is like the 'reverse' of writing the formulas. You start with the formula, and develop the name from it. The rules for this are summarized in the table below (also see pg.245):

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