University:
Bradley University
Course:
CHM 100 | Fundamentals of General Chemistry
Academic year:

2021
Views:

424

Pages:

2
Author:

Cale Mejia

When writing a formula for an ionic compound, we must first identify the symbols and ionic charges of the ions involved. • The cation (+ ion) always comes first and the anion (- ion) always comes last • If the anion ends in ate or ite, the ion is polyatomic and its symbol and charge can be found on your polyatomic ion sheet. • If the name ends in –ide, the ion is monatomic and comes from the periodic table (the only exceptions are cyanide and hydroxide, which are polyatomic) Ex. calcium nitride: Ca2+ N3- Ionic compounds are electronically neutral, so we must determine how many of each ion are needed for the charges to cancel. The easiest way to do this is to criss-cross the numerical part of the charge (not the + or – part) to become the subscript on the opposite atom. The subscript tells us how many of each ion is present. Ex. calcium nitride: Ca2+ N3- = Ca3N2 Since an ionic compound represents the LOWEST whole-number ratio of ions in a compound, we must reduce any charges that are the same in numerical value but opposite in sign. Ex. magnesium oxide: Mg2+ O2- = MgO not Mg2O2 If a polyatomic ion is present, the rules for formula writing are the same the only extra rule is:  If a polyatomic ion gets a subscript, you must place parenthesis around the polyatomic ion’s formula before writing the subscript Ex. magnesium nitrate: Mg2+ NO31- = Mg(NO3)2 not MgNO32 The parenthesis must be used in this case because you have an Mg ion and two NO3 ions NOT an Mg, an N, and 32 O’s However, you should NOT use parenthesis if there is no subscript for the polyatomic ion. For example, KNO3 should NOT be written as K(NO3)