Review of Writing Formulas and Determining Oxidation Numbers (Charges)
I. Determining Oxidation Numbers (Charges)
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Know the ones that don’t change: G1 = +1, G2 = +2, G13 = +3, G14 = +4, G15 = -3, G16 = -2, G17 = -1
Charges for transition elements (G3-G12 and the metals in other groups such as Sn & Pb) must be
determined from the formula of the compound that contains them:
o Remember positive is on the left, negative is on the right.
o Determine the total negative charge by multiplying the number of atoms of the negative element
or the number of ions of the negative polyatomic ion by the charge.
o The total positive charge must equal the total negative charge.
o Divide the total positive charge by the number of atoms of the positive element.
o The answer is the charge on an individual atom of the element in question.
Examples:
AgCl: one Cl atom times -1 give a total negative charge of -1; total positive equals +1; +1 divided by one Ag
atom equals +1
Cr(OH)3: three hydroxide ions times -1 gives a total negative charge of -3; total positive equals +3; +3 divided
by one Cr atom equals +3
Sn(CrO4)2: two chromate ions times -2 gives a total negative charge of -4; total positive equals +4; +4 divided
by one Sn atom equals +4
Cu2SO4: one sulfate ion times -2 give a total negative charge of -2; total positive equals +2; +2 divided by 2 Cu
atoms equals +1
II. Writing Formulas
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Always put the positively charged element or ion first.
If the charges have a common factor, reduced them before continuing.
The value of the charge of the positive atom or ion is the number of negative atoms or ions in the
formula.
The value of the charge of the negative atom or ion is the number of positive atoms or ions in the
formula.
More than one of a polyatomic ion must be written in parentheses.
Examples:
Na & Br: Na is +1, Br is -1; this means 1 Br atom & 1 Na atom; answer is NaBr
Ca & S: Ca is +2, S is -2; charges reduce to +1 & -1; this means 1 Ca atom & 1 S atom; answer is CaS
Fe (III) & SO4: Fe is +3, sulfate is -2; this means 2 Fe atoms & 3 sulfate ions; answer is Fe2(SO4)3
Al & OH: Al is +3, hydroxide is -1; this means 1 Al atom & 3 hydroxide ions; answer is Al(OH)3
Mn (VI) & PO3: Mn is +6, phosphate is -3; charges reduce to +2 & -1; this means 1 Mn atom & 2 phosphate
ions; answer is Mn(PO4)2