Naming Compounds and Writing Chemical Formulas

Naming Compounds and Writing Chemical Formulas

Introduction

Understanding how to name chemical compounds and write their formulas is a foundational skill in general chemistry. Compounds are classified as either ionic or covalent (molecular), and each type follows specific rules for naming and formula writing. This guide summarizes the key rules, provides examples, and includes practice problems with answers.

Ionic Compounds

Binary Ionic Compounds

Binary ionic compounds consist of a metal (cation) and a nonmetal (anion). The cation is named first, followed by the anion with its ending changed to -ide.

• Formula: Name of metal + base name of nonmetal + -ide

• Example: NaCl is named sodium chloride.

• Transition metals may require Roman numerals to indicate their charge (e.g., FeCl2 is iron(II) chloride).

Polyatomic Ionic Compounds

These compounds contain at least one polyatomic ion (a charged group of covalently bonded atoms).

• Formula: Name of cation + name of polyatomic anion

• Example: NaNO3 is sodium nitrate.

Common Polyatomic Ions

Ion	Name
CO32-	carbonate
NO3-	nitrate
PO43-	phosphate
SO42-	sulfate
ClO4-	perchlorate

Multivalent Metals

Some metals can form more than one type of positive ion. Use Roman numerals to indicate the charge.

• Example: Pb(C2H3O2)4 is lead(IV) acetate.

Covalent (Molecular) Compounds

Binary Molecular Compounds

These compounds are formed between two nonmetals. Greek prefixes are used to indicate the number of each atom present.

• Formula: (Prefix) name of first element + (prefix) base name of second element + -ide

• Do not use "mono-" for the first element.

• Example: PCl3 is phosphorus trichloride; N2O5 is dinitrogen pentoxide.

Prefix	Number
mono-	1
di-	2
tri-	3
tetra-	4
penta-	5
hexa-	6
hepta-	7
octa-	8
nona-	9
deca-	10

Acids

Binary Acids

Binary acids contain hydrogen and one other nonmetal. They are named with the prefix hydro- and the suffix -ic acid.

• Example: HCl (aq) is hydrochloric acid.

Oxoacids

Oxoacids contain hydrogen, oxygen, and another element. The name is based on the polyatomic ion:

• If the ion ends in -ate, the acid ends in -ic acid.

• If the ion ends in -ite, the acid ends in -ous acid.

• Example: HNO3 is nitric acid; HNO2 is nitrous acid.

Practice Problems and Answers

Ionic Compounds

Formula	Name
KCl	potassium chloride
CaF2	calcium fluoride
Na2O	sodium oxide
LiMnO4	lithium permanganate
BeCrO4	beryllium chromate
NH4OH	ammonium hydroxide
NaNO3	sodium nitrate
ZnSO3	zinc sulfite
Ca(ClO4)2	calcium perchlorate
MgBrO	magnesium hypobromite
Pb(C2H3O2)4	lead(IV) acetate
CuSO4·5H2O	copper(II) sulfate pentahydrate

Covalent Compounds

Formula	Name
PCl3	phosphorus trichloride
N2O5	dinitrogen pentoxide
H2S	hydrogen sulfide
HCl (aq)	hydrochloric acid
HI (aq)	hydroiodic acid
HNO3	nitric acid
HNO2	nitrous acid
H2CO3	carbonic acid
C3H8	propane
C4H8	butene

Summary Table: Key Naming Rules

Type	Naming Rule	Example
Binary Ionic	Metal + nonmetal (-ide)	NaCl: sodium chloride
Polyatomic Ionic	Metal + polyatomic ion	NaNO3: sodium nitrate
Multivalent Metal	Metal (Roman numeral) + nonmetal	FeCl2: iron(II) chloride
Binary Molecular	Prefix + nonmetal + prefix + nonmetal (-ide)	CO2: carbon dioxide
Binary Acid	Hydro- + nonmetal + -ic acid	HCl: hydrochloric acid
Oxoacid	Polyatomic ion root + -ic/-ous acid	HNO3: nitric acid

Additional info:

• When writing formulas, ensure the total positive and negative charges balance to zero.

• Hydrates are named by adding a Greek prefix for the number of water molecules (e.g., pentahydrate for 5 H2O).

• Organic compounds such as alkanes, alkenes, and alkynes follow specific IUPAC rules not detailed here.