Ionic Compound (Salt) Formulas

The following Seven Rules show how to write the chemical formula of any chemical compound which contains ions (commonly called salts) given the name of the salt. With practice, you will be able to write the formula for any salt in seconds.  Of course, you have to be able to identify the ions, so take a look at the list of Polyatomic Ions.  You may also want to look at the tutuorial on Lewis Diagrams, with some examples, and also some 3-dimensional molecular models of some of these ions.

Rule 1
Two Kinds of ions are always found in a salt:
Cation: a positively charged monatomic or polyatomic ion (see Cations)
Anion: a negatively charged monatomic or polyatomic ion (see Anions)
Rule 2
The Chemical Formula is always named or written with cations first, anions last (see Formulas (I))
Rule 3
A salt is always Neutral, which means that the total charge on the cations must equal the total charge on the anions.  To ensure this, use the cross-multiply rule.  The four steps necessary to write an initial salt formula are as follows:
Step 1: Identify the formula and charge of the anion, and the formula and charge of the cation (many of these just have to be looked up in a reference or memorized);
Step 2: Write the complete formula (including charge) of the cation and anion in square brackets [ ];
Step 3: Use the numerical value of the cation charge as the anion subscript (if greater than 1);
Step 4: Use the numerical value of the anion charge as the cation subscript (if greater than 1).
See Iron(II) Phosphate as an example.
Rule 4
Erase the ionic charges from the formula derived from Rule 3 - salt formulas must never show charges. (see Erase Charges)

Rule 5
Change all square brackets to parentheses (), because conventional chemical formulas only contain parentheses. (see Convert Brackets)

Rule 6
Remove unnecessary parentheses.  Parentheses are only necessary to separate subscripts. (see Remove).

Rule 7
Reduce to the Smallest Whole Number Ratios.  This is often unnecessary since the formula derived from the above six rules often contains the smallest whole number ratios of cations and anions.  However, consider vanadium(IV) oxide; application of the first six rules leads to the following formula:
[V4+][O2-] --> [V4+]2[O2-]4 --> [V]2[O]4 --> (V)2(O)4 --> V2O4
This formula states that the ratio of vanadium ions to oxide ions is 2:4 = 1:2, and the conventional formula would be written with the smallest whole number atomic ratios:
V2O4 = VO2
Care must be take, when reducing to the smallest whole number ratios, that the ions retain their correct identity.  Consider the following two examples:

Sodium Peroxide:

[Na+][O22-] --> [Na+]2[O22-]1 --> Na2O2, not NaO (the latter formula implies the oxide ion, O2-, which in turn implies the ion Na2+, which does not exist).
Mercury(I) Chloride:
[Hg22+][Cl-] --> [Hg22+]1[Cl-]2 --> Hg2Cl2, not HgCl (the latter formula implies the ion Hg+, which does not exist).

Further examples of salt formulas are given below.
 
 

Examples


Cations

Monatomic: Na+, H+, Ca2+, Fe2+, Al3+, Mn5+
Polyatomic: NH4+, H3O+

Anions
Monatomic: O2-, Cl-, N3-
Polyatomic: OH-, SO42-, PO43-

Formulas (I)
NaCl = [Na+][Cl-] (the square brackets do not belong in the formula, but are used to show what the ions are)
(NH4)3PO4 = [NH4+]3[PO43-]

Iron(II) phosphate
The salt named iron(II) phosphate contains the following ions:
"iron(II)" = Fe2+
"phosphate" = PO43-
Start the formula by writing the two ions is square brackets (cation first):
[Fe2+][PO43-] (don't forget the charges!!!)
The numerical value of the cation charge (+2) is 2; use this as the subscript for the anion:
[Fe2+][PO43-]2
The numerical value of the anion charge (-3) is 3; use this as the subscript for the cation:
[Fe2+]3[PO43-]2
Thus, iron(II) phosphate contains three cations for every pair of anions. This "cross-multiplication" ensures electrical neutrality, since the total positive charge on the three cations is 3(+2) = +6, the total negative charge on the two anions is 2(-3) = -6, and the total charge is +6-6 = 0.

Erase Charges
Convert the initial formula into a more conventional formula by removing the charges on the cations and anions:
 [Fe2+]3[PO43-]2 --> [Fe]3[PO4]2

Convert Brackets
Change all square brackets to parentheses (), because conventional chemical formulas only contain parentheses:
 [Fe]3[PO4]2 -->  (Fe)3(PO4)2

Remove
Remove any unnecessary parentheses.  These are parentheses which, if removed, do not make the formula ambiguous or incorrect.  Thus, "(Fe)3" can be written "Fe3", because this still indicates that there are three iron ions in the formula.  However, "(PO4)2" could not be written "PO42", because this would indicate that there are 42 O atoms for every P atom, which is certainly not true. Thus iron(II) phosphate is:
Correct: (Fe)3(PO4)2 --> Fe3(PO4)2
Incorrect: (Fe)3(PO4)2
Incorrect: (Fe)3PO42
Incorrect: Fe3PO42
Further Examples
Salt
Formula Development
Comments
 calcium nitrite
[Ca2+][NO2-]
[Ca2+]1[NO2-]2
[Ca][NO2]2
Ca(NO2)2
 Sometimes the hardest part is remembering the formula (including charge) of an ion!
The subscript "1" is understood when no subscript is written. 
 ammonium acetate
 [NH4+][C2H3O2-]
[NH4+]1[C2H3O2-]1
[NH4][C2H3O2]
NH4C2H3O2
 The subscript "1" is not written.
 magnesium phosphite
[Mg2+][PO33-]
[Mg2+]3[PO33-]2
[Mg]3[PO3]2
Mg3(PO3)2
 Don't confuse phosphite and phosphate
 manganese(IV) sulfate
 [Mn4+][SO42-]
[Mn4+]2[SO42-]4
[Mn]2[SO4]4
Mn(SO4)2
 Don't confuse manganese with magnesium.
Don't forget to reduce to the small whole number ratio cation:anion
 methylammonium perchlorate
 [CH3NH3+][ClO4-]
CH3NH3ClO4
 The methylammonium ion is not one we normally memorize.  You will be able to remember it after you study organic nomenclature.