Writing Formulas (Criss-cross Method)

Sarah L.

asked • 02/03/21

Writing Formulas of Ternary Ionic Compounds with Polyatomic Ions

The crisscross method that we used to determine the formulas of binary ionic compounds can also be used for polyatomic compounds. Remember that these compounds consist of polyatomic ions that contain more than one atom. For example, calcium phosphate is an ionic compound that has a polyatomic anion. In this compound, the calcium cation has a 2+ charge. One phosphorus (P) atom and four oxygen (O) atoms bond to form a phosphate (PO ₄ ³⁻ ) ion, which has a 3− charge.

To write the formula of calcium phosphate, move the charge of the calcium ion to the subscript position of the phosphate ion, and move the charge of the phosphate ion to the subscript position of the calcium ion. We must add parentheses around the phosphate ion to indicate the number of polyatomic ions in the compound. So, we get the following expression:

Ca ₃ (PO ₄ ) ₂

_{Can you please help, I don't understand this. I'm not sure how they got the expression.}

1 Expert Answer

Anthony T. answered • 02/04/21

Patient Math & Science Tutor

In Ca₃(PO₄)₂ calcium has a 2+ charge and phosphate has a 3- charge. The overall compound must be electrically neutral. The way to achieve this is to have 3 calcium ions (total charge 3 x 2+ = 6+) to unite with 2 phosphate ions (total charge 2 x 3- = 6-). The total positive charges equals the total negative charges, so the compound is neutral. The crisscross method is a simple way to get the formula for a binary compound consisting of ions of numerically different charges. As the phosphate ion behaves as a unit, the crisscross method works for this also.

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