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Writing Formulas and Transition Metals Nomenclature for AP Chemistry

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By — McGraw-Hill Professional
Updated on Feb 8, 2011

Practice problems for these concepts can be found at:

Writing Formulas

To write the formula from the name of a binary compound containing only nonmetals, simply write the symbols for the separate atoms with the prefixes converted to subscripts.

In all compounds, the total charge must be zero. There are NO exceptions. Thus, to determine the formula in those cases where no prefixes are given, it is necessary to have some idea what the individual charges are. The species with the positive charge is listed and named first; this is followed by the species with the negative charge. Subscripts may be needed to make sure the sum of the charges (valances) will equal zero. Examples:

  1. Magnesium oxide
  2. This gives MgO.

  3. Sodium oxide
  4. This gives Na2O.

  5. Aluminum oxide
  6. This gives Al2O3.

If a polyatomic ion must be increased to achieve zero charge, parentheses should be used. An example of this is shown as:

This gives (NH4)2SO4.

One way of predicting the values of the subscripts is to crisscross the valences. This is not a rule of nomenclature, but for practice purposes in this exercise it will be referred to as the crisscross rule. It works most of the time and therefore is worth considering. Example:

Al3+O2– crisscross the 2 from the oxygen charge to the aluminum and the 3 from the aluminum charge to the oxygen

Al23+O3–2

If the crisscross rule is applied, you should reduce the formula if possible. For example:

    Mn4+O2 crisscrosses to Mn2O4, which reduces to MnO2

If a formula is given, the crisscross rule can be reversed to give the valences:

    Al2O3
    Al23+O32–

As a first approximation, the valences of the representative elements can be predicted from their position on the periodic table. Hydrogen and the metals have positive charges beginning with +1 on the left and increasing by one as you proceed to the right on the periodic table (skipping the transition metals). Nonmetals begin with 0 in the rightmost column of the periodic table and decrease by 1 as you move to the left on the periodic table. Metalloids may be treated as metals or nonmetals. Examples are:

    Na+ Al3+ Pb4+ N3– Se2– I
    Na+ Mg2+ Al3+ Si4+ P3– S2– Cl Au0

Transition Metals

Many transition metals and the group of six elements centered around lead on the periodic table commonly have more than one valence. The valence of these metals in a compound must be known before the compound can be named. Modern nomenclature rules indicate the valence of one of these metals with a Roman numeral suffix (Stock notation). Older nomenclature rules used different suffixes to indicate the charge. Examples:

  1. FeCl3
  2. Fe3+Cl31– (crisscross rule)

    The compound is named iron(III) chloride or ferric chloride.

  3. FeCl2
  4. If chloride is –1, two chloride ions are –2. Fe has a valence of +2, to give a total charge of zero. The name is iron(II) chloride or ferrous chloride.

  5. MnO2
  6. Mn4+ (found previously)

    The name would be manganese(IV) oxide, although it is often named manganese dioxide.

The Roman numeral suffix is part of the name of the metal. Thus iron(III) is one word.

Stock notation should be used for all metals that have a variable valence. This includes almost all the transition elements and the elements immediately around lead on the periodic table. Stock notation is often omitted for Zn, Cd, and Ag, as they do not have variable valence.

Practice problems for these concepts can be found at:

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