Final answer:
To determine the correct ionic formula, count the ions in the crystal, then use the crisscross method to write the formula, ensuring the charges balance to zero. For balancing chemical equations, write down the correct formulas and balance each element by adjusting coefficients.
Step-by-step explanation:
To determine the correct ionic formula for a compound, you will need to consider the charges on the ions that make up the crystal. You've mentioned the need to 'rotate the crystal and count the number of ions', which may be relevant if you're working with a physical model, a computer simulation, or a diagram of an ionic crystal structure. Once you have the correct counts of cations (positively charged ions) and anions (negatively charged ions), you can use those to deduce the correct formula.
Writing the Chemical Formula
You can follow these steps to write accurate formulas:
Use the crisscross method to transfer the numerical values of the ions' charges to become the subscripts of the opposite ion.
Transpose the number of the positive charge to the anion and the negative charge to the cation.
Reduce these numbers to the lowest whole number ratio for the correct formula.
Make sure that the sum of the charges in the formula unit is zero.
For instance, the crystal structure of an ionic compound such as iron (III) chloride (FeCl3) reflects that there are three chloride ions for every one iron (III) ion.
Balancing Chemical Equations
If you are instead looking to balance a chemical equation, here are the steps:
Write down the correct formulas for all reactants and products.
Count the number of atoms of each element in the reactants and products.
Balance each element by adding coefficients.
Start with elements that appear in only one formula on each side of the equation if possible.
After balancing the molecular equation, you can write the ionic equation, followed by the net ionic equation, eliminating spectator ions.