1. Identify the ions:
* Write down the symbols of the cation and anion, including their charges. For example, if you're combining sodium (Na+) and chloride (Cl-), you would write:
* Na+
* Cl-
2. Crisscross the charges:
* The numerical value of the cation's charge becomes the subscript for the anion, and vice versa.
* If the charges are the same (e.g., both +2 or both -3), they cancel out, and you don't need to write a subscript.
3. Simplify the formula:
* If the subscripts have a common factor, reduce them to the simplest whole-number ratio.
Example:
Let's find the formula for the compound formed by calcium (Ca²⁺) and phosphate (PO₄³⁻).
1. Identify the ions:
* Ca²⁺
* PO₄³⁻
2. Crisscross the charges:
* The charge of Ca²⁺ (2) becomes the subscript for PO₄³⁻, giving us PO₄³⁻₂.
* The charge of PO₄³⁻ (3) becomes the subscript for Ca²⁺, giving us Ca³⁺.
* This gives us the formula: Ca₃(PO₄)₂
3. Simplify the formula:
* The formula is already in its simplest form.
Therefore, the chemical formula for the compound formed by calcium and phosphate is Ca₃(PO₄)₂.
Key Points:
* The crisscross method only works for ionic compounds, which are formed by the electrostatic attraction between oppositely charged ions.
* The subscripts in the chemical formula represent the number of each ion in the compound.
* The formula is simplified by dividing the subscripts by their greatest common factor.
Note: The crisscross method is a useful tool for remembering how to write chemical formulas, but it's important to understand the underlying concepts of ionic bonding and chemical formulas.
