1.8 Valence Electrons and Ionic Compounds

Cations and anions form via electron transfer

Metals tend to lose valence electrons to form cations (positive ions), while nonmetals tend to gain valence electrons to form anions (negative ions). Elements gain or lose electrons to achieve a stable, noble gas electron configuration. The number of electrons lost or gained typically matches the element's group number's distance from the nearest noble gas.

Predicting ionic charges from position

Group 1 metals form 1+ ions, Group 2 form 2+ ions, and Group 13 often forms 3+ ions. Group 17 nonmetals form 1− ions, Group 16 form 2− ions, and Group 15 form 3− ions. Transition metals often form multiple possible cation charges since d-electrons are less predictable.

Ionic compounds and lattice energy

Ionic compounds form when cations and anions attract each other in a repeating 3D arrangement called a crystal lattice, held together by strong electrostatic forces. Lattice energy is the energy released when gaseous ions combine into a solid lattice; it increases with greater ionic charge and decreases with larger ionic radius (Coulomb's law).

Key terms

Cation

A positively charged ion formed when an atom (typically a metal) loses one or more valence electrons.

Anion

A negatively charged ion formed when an atom (typically a nonmetal) gains one or more valence electrons.

Lattice energy

The energy released when gaseous cations and anions combine to form a solid ionic crystal lattice; increases with higher charge and smaller ionic radius.

Noble gas configuration

A stable electron arrangement matching that of the nearest noble gas, achieved by atoms gaining or losing valence electrons.

Ionic compound

A compound formed by the electrostatic attraction between oppositely charged cations and anions arranged in a repeating crystal lattice.

Try an AP-style question

What is the formula of the compound formed by Al and O?

1. Al₂O₃
2. Al₃O₂
3. AlO₂
4. AlO