Naming and Writing Formulas for Ionic and Covalent Compounds

Naming and Writing Formulas for Ionic and Covalent Compounds

Introduction

Understanding how to name and write formulas for chemical compounds is a foundational skill in GOB Chemistry. Compounds are classified as either ionic or covalent based on the types of elements involved and the nature of their bonding. This guide summarizes the rules and provides examples for naming and writing formulas for both types of compounds.

Ionic Compounds

Definition and Characteristics

• Ionic compounds are formed from the electrostatic attraction between cations (positively charged ions, usually metals) and anions (negatively charged ions, usually nonmetals).

• They typically consist of a metal and a nonmetal.

• The overall charge of the compound must be neutral.

Naming Ionic Compounds

• Binary ionic compounds (containing only two elements): Name the cation first, then the anion with the suffix -ide.

• If the metal can form more than one charge (transition metals), indicate its charge with Roman numerals in parentheses.

• For compounds containing polyatomic ions, use the name of the polyatomic ion as is.

Examples:

• NaBr: Sodium bromide

• FeCl3: Iron (III) chloride

• CaCO3: Calcium carbonate

• NH4NO3: Ammonium nitrate

Writing Formulas for Ionic Compounds

• Write the symbol for the cation followed by the symbol for the anion.

• Balance the charges so the total positive and negative charges are equal.

• Use subscripts to indicate the number of each ion needed for charge balance.

Examples:

• Potassium nitrite: KNO2

• Magnesium sulfate heptahydrate: MgSO4·7H2O

• Iron (III) phosphate: FePO4

Covalent (Molecular) Compounds

Definition and Characteristics

• Covalent compounds are formed by the sharing of electrons between nonmetal atoms.

• They typically consist of two or more nonmetals.

Naming Covalent Compounds

• Use prefixes to indicate the number of each type of atom present:

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

• The first element keeps its name; the second element gets the -ide ending.

• The prefix mono- is usually omitted for the first element.

Examples:

• CO2: Carbon dioxide

• N2O5: Dinitrogen pentoxide

• SF6: Sulfur hexafluoride

• P4O10: Tetraphosphorus decoxide

Writing Formulas for Covalent Compounds

• Use the prefixes in the name to determine the number of each atom.

• Write the symbols for the elements in the order given in the name.

• Use subscripts to indicate the number of atoms.

Examples:

• Dinitrogen trioxide: N2O3

• Phosphorus pentachloride: PCl5

• Carbon tetrachloride: CCl4

Polyatomic Ions

Common Polyatomic Ions

Ion Name	Formula	Charge
Ammonium	NH4+	+1
Nitrate	NO3-	-1
Sulfate	SO42-	-2
Phosphate	PO43-	-3
Hydroxide	OH-	-1
Carbonate	CO32-	-2

Hydrates

Naming Hydrates

• Hydrates are ionic compounds that include water molecules in their structure.

• Name the ionic compound first, then add the appropriate prefix for the number of water molecules, followed by "hydrate".

Example:

• CuSO4·5H2O: Copper(II) sulfate pentahydrate

Practice Problems and Solutions

Sample Naming Problems

• NaBr: Sodium bromide

• Fe(PO4)2: Iron(III) phosphate

• CO2: Carbon dioxide

• NH4NO3: Ammonium nitrate

• SnS2: Tin(IV) sulfide

• Ag3PO4: Silver phosphate

• Zn(NO3)2: Zinc nitrate

• MgSO4·7H2O: Magnesium sulfate heptahydrate

• CCl4: Carbon tetrachloride

Sample Formula Writing Problems

• Potassium carbonate: K2CO3

• Nickel(II) sulfide: NiS

• Phosphorus pentachloride: PCl5

• Disulfur dichloride: S2Cl2

• Gallium oxide: Ga2O3

Summary Table: Prefixes for Covalent Compounds

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

Key Equations and Concepts

• Charge balance in ionic compounds:

$\text{Total positive charge} + \text{Total negative charge} = 0$

• General formula for binary ionic compounds:

$\text{M}^{n+} + \text{X}^{m-} \rightarrow \text{M}_x\text{X}_y$

• General formula for covalent compounds (using prefixes):

$\text{Prefix}_1\text{Element}_1\ \text{Prefix}_2\text{Element}_2$

Additional info: These notes expand on the practice worksheets by providing rules, examples, and tables for naming and writing formulas for both ionic and covalent compounds, including polyatomic ions and hydrates, as relevant to GOB Chemistry.