Ionic Compounds — Lecture 14 Summary Summary & Study Notes

⚛️ Overview

Ionic compounds form when atoms transfer electrons so each atom achieves a filled electron shell like a nearby noble gas. The resulting positively charged cations and negatively charged anions pack into a repeating crystal lattice rather than existing as discrete molecules. Common examples include $NaCl$ and $MgO$.

🔋 Filled e‑Shields are Stable

Atoms form ions to reach a stable electronic configuration (a filled valence shell). This drive toward a noble‑gas electron configuration explains why metals tend to lose electrons and nonmetals tend to gain them.

🔌 Metals vs Nonmetals

Metals generally lose electrons to form cations (e.g., $Na^+$). Nonmetals generally gain electrons to form anions (e.g., $Cl^-$). The resulting electrostatic attraction between oppositely charged ions produces the ionic bond.

🏷️ Naming ionic compounds — predictable metal charge

For metals with a predictable (fixed) charge (often main‑group metals): the naming rule is Name of metal + name of anion + -ide. For example, $NaCl$ is named sodium chloride. When the anion is a polyatomic ion that contains oxygen, the polyatomic name is used (see oxyanion section).

Example with a polyatomic anion: $Na_2SO_4$ is sodium sulfate (the sulfate ion is $SO_4^{2-}$).

🧭 Naming ionic compounds — variable metal charge

When a metal can have multiple possible charges (commonly transition metals), indicate the metal’s charge with a Roman numeral in parentheses after the metal name. The rule is Name of metal (Roman numeral for charge) + name of anion + -ide (or polyatomic name).

Example: $FeCl_3$ is iron(III) chloride because iron is in the $+3$ oxidation state in this compound ($Fe^{3+}$).

✳️ Oxyanions and the -ate / -ite endings

Many common anions that contain oxygen are called oxyanions. They frequently use the endings -ate (more oxygen) and -ite (one fewer oxygen). Examples: sulfate $SO_4^{2-}$ and sulfite $SO_3^{2-}$. Prefixes per- and hypo- indicate even more or fewer oxygens, respectively.

🧾 Binary anions and -ide

Simple (binary) anions composed of a nonmetal and electrons use the -ide suffix (for example, $Cl^-$ → chloride, $O^{2-}$ → oxide). Binary ionic compounds use these -ide names for the anion.

🔬 Formula unit (Ionic compounds)

A formula unit is the simplest whole‑number ratio of ions in an ionic solid that yields electrical neutrality. Ionic solids are extended lattices, so there are no discrete molecules; we report the formula unit instead. Examples: $NaCl$ (1:1 ratio), $Mg_3(PO_4)_2$ (3 magnesium ions per 2 phosphate ions).

Note: the course notes included an example written as $M(g)_2(PO_4)_2$ — ensure you check the intended metal and subscripts when interpreting formulas.

✅ Key points to remember

• Filled valence shells (noble gas configurations) drive ion formation.
• Metals → cations, nonmetals → anions.
• Use -ide for simple binary anions; use polyatomic names (e.g., sulfate) for oxyanions.
• Use Roman numerals for metals with variable charges.
• Ionic solids are described by their formula unit, not molecular formulas.