1. Subscripts:
* Cations (positively charged ions) are written first in the formula.
* Anions (negatively charged ions) are written second in the formula.
* Subscripts indicate the number of each ion needed to balance the charges and create a neutral compound.
2. Charges:
* The charges of each ion are not explicitly written in the final formula, but they are used to determine the subscripts.
Example:
Consider the compound sodium chloride (NaCl).
* Sodium (Na) has a +1 charge (Na⁺).
* Chloride (Cl) has a -1 charge (Cl⁻).
To balance the charges, we need one sodium ion (Na⁺) and one chloride ion (Cl⁻). Therefore, the formula for sodium chloride is NaCl.
More Complex Example:
Consider the compound magnesium chloride (MgCl₂).
* Magnesium (Mg) has a +2 charge (Mg²⁺).
* Chloride (Cl) has a -1 charge (Cl⁻).
To balance the charges, we need one magnesium ion (Mg²⁺) and two chloride ions (2Cl⁻). Therefore, the formula for magnesium chloride is MgCl₂.
Key Points:
* The subscripts in a formula indicate the ratio of ions in the compound.
* The charges of the ions are used to determine the subscripts.
* The sum of the charges of the cations must equal the sum of the charges of the anions for a neutral compound.
Important Note:
* The charges of common ions are often memorized or found on a periodic table or a chart of common ions.
* You can use criss-cross method to help you determine the subscripts: Write the charges of the ions and then switch the numbers.
