Writing Binary Ionic Compound Formulas, Schemes and Mind Maps of Chemistry

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Writing Binary Ionic

Compound Formulas

THIS PRESENTATION WILL EXPLAIN HOW TO WRITE THE FORMULAS FOR BINARY

IONIC COMPOUNDS.

A binary ionic

compound contains an

ion that is a metal

( cation ) and an ion that

is a nonmetal ( anion ).

IONIC COMPOUND = METALLIC ION + NONMETALLIC ION

Determining Charge on Anions

 (1) A nonmetallic element’s placement on the periodic table will give you an indication as to the charge on the ion.  Ions of the halogen elements (Group 7A) are always 1-. For example, the fluoride ion is written F^1 -.  Ions formed from the first three elements in group 6A always form 2- ions. For example, the oxide ion is O^2 -^ and the sulfide ion is always S^2 -.  Ions formed from the first two elements in group 5A always form 3- ions. Those ions would be the nitride ion (N^3 - ) and the phosphide ion (P^3 - ).  (2) Note that all of the anions end in the suffix – ide.

Determining Binary Ionic Compound

Formulas

 The guiding principle when writing binary ionic compound formulas is that the positive charges on the cation must balance the negative charges on the anion.  Examples: For Na1+^ and Cl^1 - ,^ the resulting formula would be NaCl (1 positive charge balanced with one negative charge); for Ca2+^ and I^1 - , the resulting formula would be CaI 2 because you will need 2 negative charges from iodide ions to balance the positive 2 charge on the calcium ion.  One easy way to determine the formula if the charges do not cancel out is to use the “criss-cross” method.  (1) Write the cation first with its charge  (2) Write the anion second with its charge.  (3) Drop the “+” and “-” signs and bring the number of the charge down as a subscript for the other element. The result should be the correct formula for that compound.