Writing Formulas for Binary Ionic Compounds

Fundamentals of Ionic Compound Formulas

• Ionic compounds are composed of positive ions (cations) and negative ions (anions) held together by mutual attraction.
• Chemical formulas list the symbol of the cation first, followed by the anion.
• Subscripts indicate the number of each ion required to achieve a neutral formula unit (where total positive and negative charges sum to zero).
• In $MgCl_2$, one $Mg^{2+}$ ion (total positive charge of $+2$) balances two $Cl^-$ ions (total negative charge of $-2$).

The Crisscross Method and Subscript Reduction

• Use the absolute value of the cation's charge as the anion's subscript and the absolute value of the anion's charge as the cation's subscript.
• Subscripts must be reduced to the smallest whole number ratio.
• Example: Combining Manganese ($+4$) and Sulfide ($-2$) initially gives $Mn_2S_4$, which reduces to $MnS_2$.
• Example: Combining Iron ($+3$) and Iodide ($1-$) results in $FeI_3$.

Exceptions to Subscript Reduction

• Certain ions exist as bonded pairs and are never reduced in formulas:   - Mercury (I): $Hg_2^{2+}$   - Peroxide: $O_2^{2-}$

Determining Ion Charges from the Periodic Table

• Element position on the periodic table determines common ionic charges (e.g., Calcium forms $Ca^{2+}$, Phosphorus forms Phosphide $P^{3-}$).
• Transition metals capable of multiple charges are designated with Roman numerals, such as Chromium (II) for $Cr^{2+}$.
• Example: Calcium ($Ca^{2+}$) and Phosphorus ($P^{3-}$) cross to form calcium phosphate, $Ca_3P_2$.
• Example: Chromium (II) ($Cr^{2+}$) and Bromide ($Br^-$) form $CrBr_2$.
• Example: Potassium ($K^+$) and Nitrogen ($N^{3-}$) form potassium nitride, $K_3N$.