Ionic Compounds: Structure, Formation, and Properties

Chapter 3: Ionic Compounds

3.1 Ions

Ions are charged particles formed when atoms gain or lose electrons. They play essential roles in biological and chemical processes, including nerve signal transmission.

• Cation: A positively charged ion formed when a neutral atom loses one or more electrons.

• Anion: A negatively charged ion formed when a neutral atom gains one or more electrons.

• Example: Sodium (Na) loses one electron to form Na+; chlorine (Cl) gains one electron to form Cl-.

• Properties of Ionic Compounds (e.g., NaCl):

  • High melting points

  • Stable, white, crystalline solids

  • Soluble in water

  • Conduct electricity when dissolved in water (due to free-moving ions)

3.2 Ions and the Octet Rule

The octet rule states that main group elements tend to react to achieve eight valence electrons, resembling the electron configuration of noble gases.

• Metals lose electrons to form cations with noble gas configurations (preceding noble gas).

• Nonmetals gain electrons to form anions with noble gas configurations (following noble gas).

• Example (Magnesium):

  • Electron configuration: $1s^2\ 2s^2\ 2p^6\ 3s^2$

  • Mg loses two electrons to form Mg2+ with a neon configuration: $1s^2\ 2s^2\ 2p^6$

• Example (Nitrogen):

  • Electron configuration: $1s^2\ 2s^2\ 2p^3$

  • N gains three electrons to form N3- with a neon configuration: $1s^2\ 2s^2\ 2p^6$

3.3 Ions of Some Common Elements

The periodic table can be used to predict the charges of ions formed by main group elements.

• Group 1A: Form +1 cations (e.g., Na+)

• Group 2A: Form +2 cations (e.g., Ca2+)

• Group 6A: Form –2 anions (e.g., O2-, S2-)

• Group 7A (Halogens): Form –1 anions (e.g., Cl-)

• Transition Metals: Can form cations with multiple possible charges (octet rule not strictly followed)

• Charge Prediction:

  • For metals: cation charge = group number

  • For nonmetals: anion charge = 8 – group number

3.4 Periodic Properties and Ion Formation

Ion formation is influenced by periodic trends such as ionization energy and electron affinity.

• Ionization Energy: Energy required to remove an electron from a gaseous atom. Lower values mean easier cation formation (e.g., alkali metals).

• Electron Affinity: Energy released when an atom gains an electron. Higher values mean easier anion formation (e.g., halogens).

• Trends:

  • Metals (left side) lose electrons easily (low ionization energy).

  • Nonmetals (right side) gain electrons easily (high electron affinity).

  • Noble gases are generally unreactive.

3.5 Naming Monoatomic Ions

Monoatomic ions are named based on their element and charge.

• Main Group Metal Cations: Name = element + "ion" (e.g., potassium ion, magnesium ion)

• Transition Metal Cations:

  • Old system: -ous (lower charge), -ic (higher charge) (e.g., ferrous ion, ferric ion)

  • New system: Roman numeral in parentheses (e.g., iron(II) ion, iron(III) ion)

• Anions: Replace the ending with "-ide" + "ion" (e.g., chloride ion, oxide ion)

Table: Names of Some Transition Metal Cations

Element	Symbol	Old Name	New Name
Iron	Fe2+	Ferrous ion	Iron(II) ion
Iron	Fe3+	Ferric ion	Iron(III) ion
Copper	Cu+	Cuprous ion	Copper(I) ion
Copper	Cu2+	Cupric ion	Copper(II) ion
Chromium	Cr2+	Chromous ion	Chromium(II) ion
Chromium	Cr3+	Chromic ion	Chromium(III) ion
Mercury*	Hg22+	Mercurous ion	Mercury(I) ion
Mercury	Hg2+	Mercuric ion	Mercury(II) ion

*This cation is composed of two mercury atoms, each with an average charge of +1.

Table: Names of Some Common Anions

Element	Symbol	Name
Fluorine	F-	Fluoride ion
Chlorine	Cl-	Chloride ion
Bromine	Br-	Bromide ion
Iodine	I-	Iodide ion
Oxygen	O2-	Oxide ion
Sulfur	S2-	Sulfide ion
Nitrogen	N3-	Nitride ion
Phosphorus	P3-	Phosphide ion

3.6 Polyatomic Ions

A polyatomic ion is an ion composed of two or more atoms covalently bonded, carrying a net charge.

• Common polyatomic ions must be memorized due to their frequent occurrence.

• Examples:

  • Nitrate: NO3-

  • Sulfate: SO42-

  • Hydroxide: OH-

  • Ammonium: NH4+

Table: Some Common Polyatomic Ions

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

3.7 Ionic Bonds

Ionic bonds are the electrostatic attractions between oppositely charged ions in an ionic compound.

• Formed by electron transfer from a metal to a nonmetal.

• Result in crystalline solids with repeating patterns (lattices).

• Example: NaCl crystal consists of alternating Na+ and Cl- ions.

3.8 Formulas of Ionic Compounds

Ionic compounds are electrically neutral; their formulas reflect the lowest whole-number ratio of ions that results in zero net charge.

• Steps to Write Formulas:

  1. Identify the cation and anion (including their charges).

  2. Determine the ratio needed for total positive and negative charges to balance (sum to zero).

  3. Write the cation first, then the anion. Do not include charges in the formula.

  4. Use parentheses for polyatomic ions if more than one is needed (e.g., Ca(NO3)2).

• Formula Unit: The simplest ratio of ions in an ionic compound (e.g., NaCl, CaF2).

• Example: Calcium nitrate: Ca2+ and NO3- combine to form Ca(NO3)2.

3.9 Naming Ionic Compounds

The name of an ionic compound consists of the cation name followed by the anion name, separated by a space.

• Type I: Cations from main group elements (fixed charge). Do not specify the charge.

  • Example: NaCl = sodium chloride

  • MgCO3 = magnesium carbonate

• Type II: Cations from metals with variable charges (usually transition metals). Specify the charge with a Roman numeral.

  • FeCl2 = iron(II) chloride or ferrous chloride

  • FeCl3 = iron(III) chloride or ferric chloride

• Do not use prefixes (e.g., di-, tri-) in ionic compound names.

Table: Some Common Ionic Compounds and Their Applications

Compound	Formula	Application
Sodium chloride	NaCl	Table salt, food seasoning
Calcium carbonate	CaCO3	Antacid, chalk
Potassium iodide	KI	Source of iodine in diet
Magnesium sulfate	MgSO4	Epsom salt, laxative
Iron(III) chloride	FeCl3	Etching agent for circuit boards

3.10 Some Properties of Ionic Compounds

Ionic compounds exhibit characteristic physical properties due to the strong electrostatic forces between ions.

• High melting and boiling points (e.g., NaCl melts at 801°C, boils at 1413°C)

• Hard, brittle solids (shatter when struck)

• Conduct electricity when dissolved in water (electrolytes)

• Solubility in water varies; depends on the attraction between water molecules and ions

3.11 H+ and OH- Ions: An Introduction to Acids and Bases

The hydrogen ion (H+) and hydroxide ion (OH-) are central to acid-base chemistry.

• Acid: A substance that provides H+ ions in water. The H+ ion is a proton; in water, it forms the hydronium ion (H3O+).

• Base: A substance that provides OH- ions in water. The hydroxide ion consists of an oxygen atom covalently bonded to a hydrogen atom.

• Examples of Acids:

  • Hydrochloric acid (HCl): provides one H+ per molecule

  • Sulfuric acid (H2SO4): provides two H+ per molecule

  • Phosphoric acid (H3PO4): provides three H+ per molecule

• Examples of Bases:

  • Sodium hydroxide (NaOH): provides one OH- per formula unit

  • Barium hydroxide (Ba(OH)2): provides two OH- per formula unit

Table: Some Common Acids and the Anions Derived from Them

Acid	Formula	Anion	Anion Name
Hydrochloric acid	HCl	Cl-	Chloride
Hydrobromic acid	HBr	Br-	Bromide
Hydroiodic acid	HI	I-	Iodide
Nitric acid	HNO3	NO3-	Nitrate
Sulfuric acid	H2SO4	SO42-	Sulfate
Phosphoric acid	H3PO4	PO43-	Phosphate

Additional info: The above tables are reconstructed based on standard chemistry knowledge and the context provided in the notes. The names, formulas, and charges of ions are essential for understanding ionic compounds and their nomenclature.