Question

In the writing of ionic chemical formulas, what factor is "crossed over" in the crossover rule?

Answer 1

In the writing of ionic chemical formulas the value of each ion's charge is crossed over in the crossover rule.

Rules for naming Ionic compounds

• Frist Rule
  The cation (element with a negative charge) is written first in the name then the anion(element with a positive charge) is written second in the name.
  
• Second rule
  When the formula unit contains two or more of the same polyatomic ion, that ion is written in parentheses with the subscript written outside the parentheses.
  Example: Sodium carbonate is written as Na₂CO₃ not Na₂(CO)₃
  
• Third rule
  If the cation is a metal ion with a fixed charge then the name of the cation will remain the same as the (neutral) element from which it is derived (Example: Na+ will be sodium).
  If the cation is a metal ion with a variable charge, the charge on the cation is indicated using a Roman numeral, in parentheses, immediately following the name of the cation (example: Fe³⁺ = iron(III)).
  

• Fourth rule
  If the anion is a monatomic ion, the anion is named by adding the suffix -ide to the root of the element name (example: F = Fluoride).
  

The oxidation state of each ion is also important, thus in the crossover rule, the value of each ion's charge is crossed over.

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