3.7 Ionic and Molecular Compounds | Chemistry: Atoms First 2e

Learning Objectives

Define ionic and molecular (covalent) compounds
Predict the type of compound formed from elements based on their location within the periodic table
Determine formulas for simple ionic compounds

Core Concepts & Principles

Chemical compounds form when atoms transfer or share electrons. The nature of these electron interactions dictates whether a compound is classified as ionic or molecular.

Ions and Periodic Trends

When atoms gain or lose electrons, they form electrically charged particles called ions.

Cations (Positive Ions): Formed when metal atoms lose electrons. Main-group metals typically form positive charges equal to their group number (e.g., Group 1 forms $1^+$ , Group 2 forms $2^+$ ).
Anions (Negative Ions): Formed when nonmetal atoms gain electrons. Main-group nonmetals form negative charges equal to the number of columns left of the noble gases (e.g., Group 17 forms $1^-$ , Group 16 forms $2^-$ ).
Transition Metals: Often exhibit variable, unpredictable charges (e.g., iron can be $2^+$ or $3^+$ ).

Types of Chemical Bonding

The forces holding atoms or ions together determine the bond type:

Ionic bonds: Electrostatic forces of attraction between oppositely charged ions (cations and anions) resulting from a complete transfer of electrons.
Covalent bonds: Attractive forces between the positively charged nuclei of bonded atoms and one or more shared pairs of electrons located between them.

Ionic vs. Molecular Compounds

Ionic compounds: Formed when a metal reacts with a nonmetal via electron transfer. They are crystalline solids with high melting and boiling points. They do not conduct electricity as solids because ions are fixed, but they do conduct electricity when molten because ions can move freely.
Molecular (covalent) compounds: Formed when nonmetals combine by sharing electrons. They consist of discrete, neutral molecules and typically have lower melting and boiling points.

Predicting Compound Types from the Periodic Table

Metal + Nonmetal: Generally forms an ionic compound held together by ionic bonds.
Nonmetal + Nonmetal: Generally forms a molecular compound held together by covalent bonds.

Problem-Solving Routines & Methods

Determining Formulas for Ionic Compounds

1
Identify the cation (metal or polyatomic ion) and its charge.
2
Identify the anion (nonmetal or polyatomic ion) and its charge.
3
Determine the simplest whole-number ratio of ions so that the total positive charge equals the total negative charge.
4
Write the formula using subscripts. Enclose polyatomic ions in parentheses with an outer subscript if more than one is needed.

Pro-Tip: Never change the internal subscripts of a polyatomic ion when balancing a formula!

Electrical Neutrality Principle

\text{Total Positive Charge} = \text{Total Negative Charge}

Every ionic compound must be electrically neutral overall (net charge = 0).

Variables & Constants

\text{Net Charge}

(\text{cations} \times \text{charge}) + (\text{anions} \times \text{charge}) = 0

Practice & Concept Checks

Concept Check

What is the chemical formula of the ionic compound formed between calcium (

Ca^{2+}

) and phosphate (

PO_4^{3-}

Concept Check

Predict whether sulfur dioxide (

SO_2

) is an ionic or molecular compound, and explain why.

Key Terms & Vocabulary

monatomic ionsIons

Ions formed from only a single atom that has gained or lost electrons.

Example: Na⁺, Cl⁻, Ca²⁺

polyatomic ionsIons

Electrically charged molecules consisting of a group of bonded atoms that act as a single, discrete unit.

Example: NH₄⁺, SO₄²⁻, OH⁻

OxyanionsIons

Polyatomic ions that contain one or more oxygen atoms combined with another element.

Example: NO₃⁻ (nitrate), PO₄³⁻ (phosphate)

ionic bondsChemical Bonding

Electrostatic forces of attraction between oppositely charged cations and anions resulting from electron transfer.

Example: The attraction between Na⁺ and Cl⁻ in NaCl

covalent bondsChemical Bonding

Attractive forces between the positively charged nuclei of bonded atoms and one or more pairs of shared electrons.

Example: The bonds holding the H and O atoms together in H₂O

ionic compoundCompounds

A compound containing ions held together by electrostatic ionic bonds, typically formed by combining a metal and a nonmetal.

Example: NaCl, CaSO₄, Al₂O₃

molecular compoundsCompounds

Compounds that consist solely of discrete, neutral molecules formed when nonmetal atoms share electrons; also called covalent compounds.

Example: H₂O, CO₂, NH₃