Question

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

Answer 1

Ionic compounds are formed by the complete transfer of electrons between the atoms.  The atom which gains electron(s) forms anion whereas loss of electron(s) results in the formation of cation.  They are bonded to each other by electrostatic force of attraction between the negatively and positively charged atoms.

While writing the ionic chemical formula for binary ionic compound the rules are:

• Writing the chemical symbol of the metals and non-metals involved in the compound formation.
• The charge i.e. the absolute value of oxidation number of each metal is written on respective atoms.
• The charge i.e. the absolute value of oxidation number are cross-multiplied that is the charge of first ion becomes the subscript of second ion and vice versa.

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