To determine a formula of a compound from its name you have to identify a compound as

Ionic Compound
•Ionic compounds are made from a metal and a nonmetal.

Covalent Compound
•Covalent compounds are made from two nonmetals.

Acid
•Acids are made from H and an anion. The anion may or may not be polyatomic.

Covalent compunds: write the element with a subscript that corresponds to the prefix.

For example, carbon tetraiodide...

Carbon is a nonmetal, and iodide is a nonmetal; therefore, carbon and iodide form a covalent compound. Thus, the covalent molecule nomenclature rules are used.

Carbon is C, idodide is I, and tetra means 4, so

carbon tetraiodide is CI₄.

(hint: For our purposes, only covalent compounds use prefixes; therefore, the presence of a prefix means the compound must be covalent.)

Acids: add as many protons (H⁺) as needed to cancel the charge of the anion.

For example, hydrochloric acid...

The name has the word acid in it; therefore, it is an acid. Thus, the acid nomenclature rules are used.

Only acids based on monoatomic ions use the hydrosomething acid notation. The chloro is referring to the chloride anion, Cl^-.

One proton is needed to cancel the charge of the chloride ion, so

hydrochloric acid is HCl.

Example, phosphoric acid...

The name has the word acid in it; therefore, it is an acid. Thus, the acid nomenclature rules are used.

The name doesn't use the hydrosomething acid notation, so the phosphoric must be referring to the polyatomic ion phosphate.

Three protons are needed to cancel the charge on the phosphate ion, PO₄^3-, so

Ionic compounds can be more difficult.

For example, iron (III) oxide...

Iron is a metal and oxygen (oxide) is a nometal; therefore, iron(III) oxid is an ionic compound. Thus, the nomenclature rules for ionic compounds are used.

The charges of the cations and the anions must be known to determine the formula of the compound.

Step 1 determine the charge of the cation.

Type I. Check the periodic table.

Type II. The name of a Type II cation contains the charge!

Step 2 Determine the charge of the anion.

Polyatomic anion. Remember the charge.

Monoatomic anion. Check the periodic table.

Step 3 Combine ions so that the charges cancel.

Mg is Mg²⁺
Cl is Cl^-

MgCl₂