Names and Formulas of Ionic and Covalent Compounds

Names and Formulas of Binary Ionic Compounds

Introduction

Binary ionic compounds are composed of two different elements: a metal (which forms a cation) and a nonmetal (which forms an anion). Understanding how to determine their formulas and names is fundamental in general chemistry.

Formation of Binary Ionic Compounds

• Cations are positively charged ions formed by metals losing electrons.

• Anions are negatively charged ions formed by nonmetals gaining electrons.

• The formula of an ionic compound reflects the smallest whole-number ratio of ions that results in a net charge of zero (electrical neutrality).

Determining the Formula of Binary Ionic Compounds

• Identify the charges of the cation and anion using the periodic table.

• Balance the total positive and negative charges to achieve neutrality.

• Write the formula with the cation first, followed by the anion.

Example: If rubidium (Rb, group 1, charge +1) reacts with oxygen (O, group 16, charge -2), the formula is determined as follows:

• Rb+ and O2−

• Two Rb+ ions are needed to balance one O2− ion.

Formula: $\mathrm{Rb_2O}$

Common Binary Ionic Compounds

• Potassium oxide: $\mathrm{K_2O}$

• Magnesium bromide: $\mathrm{MgBr_2}$

• Aluminum sulfide: $\mathrm{Al_2S_3}$

Practice Questions

• What is the formula for magnesium and sulfur? Answer: $\mathrm{MgS}$ (Mg2+ and S2− combine in a 1:1 ratio)

• What is the correct formula for barium nitride? Answer: $\mathrm{Ba_3N_2}$ (Ba2+ and N3−; 3 Ba2+ for every 2 N3−)

Periodic Table and Ion Charges

The periodic table can be used to predict the charges of ions:

Group	Common Cation Charge	Common Anion Charge
1 (Alkali metals)	+1	-
2 (Alkaline earth metals)	+2	-
13	+3	-
15	-	-3
16	-	-2
17 (Halogens)	-	-1

Additional info: Transition metals may have multiple possible charges, indicated by Roman numerals in the compound name (e.g., iron(III) chloride for FeCl3).

Names and Formulas of Ionic Compounds with Polyatomic Ions

Introduction

Some ionic compounds contain polyatomic ions, which are charged groups of covalently bonded atoms. These ions act as a single unit in chemical reactions and formulas.

Common Polyatomic Ions

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

Naming Compounds with Polyatomic Ions

• Name the cation first, then the polyatomic anion.

• If more than one polyatomic ion is needed, use parentheses in the formula.

Example: Calcium hydroxide is $\mathrm{Ca(OH)_2}$.

Names and Formulas of Binary Covalent Compounds

Introduction

Binary covalent compounds are formed between two nonmetals. Their names use prefixes to indicate the number of each atom present.

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-

Naming Binary Covalent Compounds

• The first element keeps its name; the second element ends with "-ide".

• Prefixes are used for both elements, except "mono-" is usually omitted for the first element.

Example: SF2 is sulfur difluoride.

Practice: Naming and Writing Formulas

• FeCl3: iron(III) chloride

• Lead(IV) oxide: PbO2

• CuS: copper(II) sulfide

• CaF2: calcium fluoride

• CuF2: copper(II) fluoride

Summary Table: Types of Compounds and Naming Rules

Type	Components	Naming Rule	Example
Binary Ionic	Metal + Nonmetal	Metal name + nonmetal(-ide)	NaCl: sodium chloride
Ionic with Polyatomic Ion	Metal + Polyatomic ion	Metal name + polyatomic ion name	CaCO3: calcium carbonate
Binary Covalent	Nonmetal + Nonmetal	Prefix + element + prefix + element(-ide)	CO2: carbon dioxide