POLYATOMIC COMPOUNDS

POLYATOMIC COMPOUNDS Polyatomic compounds are groups of non-metal atoms that travel together. The group has an overall negative or positive charge and thus has an ionic character. All but one of the polyatomic ions are negative. Ammonia, NH3+ and Ammonium, NH4+ are the exceptions.

Naming Compounds Involving Polyatomic Ions Example 1: Write the name of the compound Na2CO3. When you write the name of the compound, make sure that you remember the anion is named according to the polyatomic ion rather than the names of the individual elements. Sodium carbonate Practice: Write the name of Ca(OH)2. Calcium hydroxide

Iron has two possible ionic charges: +2 and +3. Example 2: Write the name of the compound Fe(NO3)3. Remember Fe is one of the transitional metals, which means it has more than one ionic charge. Step 1: Write the name of the metal and check whether it has more than one possible ionic charge. If it does, go to step 2. If not, go to step 3 Iron has two possible ionic charges: +2 and +3. Step 2: Determine the ionic charge of the metal. 3+ 1- Fe (NO3)3 The ionic charge of iron in this compound is +3, indicated by the Roman numeral “III”. Step 3: Write the name of the compound, with the Roman numeral if necessary.  The name of Fe(NO3)3 is iron (III) nitrate.

Therefore, the name of CuSO4 is copper (II) sulfate. Practice: Write the name of CuSO4. +1 -1 Cu (SO4) If we multiply (SO4) by 2 we must multiply the charge for Cu by 2 also (SO4) must have a charge of -2 so we must multiply it by 2 -2 +2 Cu (SO4)   The ionic charge of copper in this compound is +2. Therefore, the name of CuSO4 is copper (II) sulfate.

Writing Formulas Involving Polyatomic Ions The crossover rule is also used when writing the formula of a compound containing a polyatomic ion. Example 1: Write the chemical formula of sodium phosphate.   Step 1: Write the symbols of each ion, beginning with the cation (metal). Na PO4 Step 2: Write the ionic charges above each ion.   Na (PO4) +1 -3 Step 3: Crisscross the numbers of the ionic charges so that they now become subscripts. (Remember to remove the positive or negative signs.) Na3 (PO4)1 Step 4: Write the chemical formula using the coefficients as subscripts. Na3 (PO4)

Hydrogen + Polyatomic ion (aq) after the formula means that it is an oxyacid – liquid. the formula you make must have the (aq) after in subscript and when you need to make a name, you know it is from the oxyacid table so the name must have “acid” after it. Hydrogen + Polyatomic ion