Chapter 3: Ionic Compounds – Structured Study Notes for GOB Chemistry

Chapter 3: Ionic Compounds

3.1 Ions

Ions are fundamental to many chemical and biological processes, including nerve signal transmission. An ion forms when a neutral atom gains or loses electrons, resulting in a charged particle.

• Cation: A positively charged ion formed by the loss of one or more electrons.

• Anion: A negatively charged ion formed by the gain of 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: High melting points, stable crystalline solids, water solubility, and electrical conductivity in solution.

3.2 Ions and the Octet Rule

The octet rule states that main group elements tend to react to achieve eight valence electrons, resembling noble gas configurations.

• Metals: Lose electrons to form cations with noble gas configurations before them in the periodic table.

• Nonmetals: Gain electrons to form anions with noble gas configurations after them.

• Example: Magnesium (Mg, Z=12) loses two electrons to form Mg2+ with a neon configuration.

• Example: Nitrogen (N, Z=7) gains three electrons to form N3- with a neon configuration.

Electron Configuration Example:

• Magnesium: $1s^2\ 2s^2\ 2p^6\ 3s^2$

• Mg2+: $1s^2\ 2s^2\ 2p^6$

• Nitrogen: $1s^2\ 2s^2\ 2p^3$

• N3-: $1s^2\ 2s^2\ 2p^6$

3.3 Ions of Some Common Elements

The periodic table helps 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: Form -1 anions (e.g., Cl-, Br-)

• Transition Metals: Can form cations of multiple 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

Periodic trends influence ion formation. Metals tend to lose electrons, nonmetals tend to gain electrons.

• Ionization Energy: Energy required to remove an electron; low for metals, high for nonmetals.

• Electron Affinity: Energy released when an atom gains an electron; high for halogens.

• Trend: Alkali metals lose electrons easily; halogens gain electrons easily; noble gases are 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).

• Transition Metal Cations: Old system: -ous (lower charge), -ic (higher charge); New system: Roman numeral (e.g., iron(II) ion).

• Anions: Element name ending replaced with "-ide" + "ion" (e.g., chloride ion).

Transition Metal	Old Name	New Name
Fe2+	Ferrous ion	Iron(II) ion
Fe3+	Ferric ion	Iron(III) ion
Cu+	Cuprous ion	Copper(I) ion
Cu2+	Cupric ion	Copper(II) ion
Hg22+	Mercurous ion	Mercury(I) ion
Hg2+	Mercuric ion	Mercury(II) ion

Anion	Name
Cl-	Chloride ion
O2-	Oxide ion
S2-	Sulfide ion
N3-	Nitride ion

3.6 Polyatomic Ions

Polyatomic ions are ions composed of multiple atoms covalently bonded, carrying a net charge.

• Examples: Nitrate (NO3-), sulfate (SO42-), ammonium (NH4+).

• These ions are common and their names and formulas should be memorized.

Polyatomic Ion	Formula	Charge
Nitrate	NO3	-1
Sulfate	SO4	-2
Phosphate	PO4	-3
Ammonium	NH4	+1
Hydroxide	OH	-1
Carbonate	CO3	-2
Acetate	CH3COO	-1

3.7 Ionic Bonds

Ionic bonds are formed by the electrostatic attraction between oppositely charged ions in ionic compounds.

• Metals transfer electrons to nonmetals, forming cations and anions.

• Ionic solids are held together by these bonds, resulting in crystalline structures.

• Example: Sodium chloride (NaCl) is held together by ionic bonds between Na+ and Cl-.

3.8 Formulas of Ionic Compounds

Ionic compounds are electrically neutral. Their formulas reflect the ratio of cations to anions needed to balance charges.

• Rule: The total positive charge must equal the total negative charge.

• Formula unit: The simplest ratio of ions in an ionic compound.

• Writing Formulas:

  • List cation first, anion second.

  • Do not write charges in the formula.

  • Use parentheses for polyatomic ions with subscripts.

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

Worked Example:

• Ca2+ + 2 NO3- → Ca(NO3)2

3.9 Naming Ionic Compounds

Ionic compounds are named by stating the cation first, then the anion.

• Type I: Cations of main group elements; charge does not vary. Example: NaCl is sodium chloride.

• Type II: Metals with variable charges; specify charge with Roman numerals or old system. Example: FeCl2 is iron(II) chloride or ferrous chloride.

• Do not use prefixes like "di-" or "tri-" for ionic compounds.

Compound	Name
NaBr	Sodium bromide
CaBr2	Calcium bromide
Na2S	Sodium sulfide
CaS	Calcium sulfide
Na2SO4	Sodium sulfate
CaSO4	Calcium sulfate
Na3PO4	Sodium phosphate
Ca3(PO4)2	Calcium phosphate

3.10 Properties of Ionic Compounds

Ionic compounds exhibit distinctive physical properties due to their ionic bonds.

• High melting and boiling points: Example: NaCl melts at 801°C, boils at 1413°C.

• Electrical conductivity: Conduct electricity when dissolved in water due to free-moving ions.

• Brittleness: Ionic solids shatter when struck.

• Solubility: Many ionic compounds dissolve in water, but not all.

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

Hydrogen cation (H+) and hydroxide anion (OH-) are central to acid-base chemistry.

• Acid: Provides H+ ions in water. H+ is a proton; in water, it forms hydronium (H3O+).

• Base: Provides OH- ions in water. OH- is a polyatomic ion with covalent O-H bond.

• Examples:

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

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

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

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

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

Acid	Formula	Anion Derived
Hydrochloric acid	HCl	Cl- (chloride)
Sulfuric acid	H2SO4	SO42- (sulfate)
Phosphoric acid	H3PO4	PO43- (phosphate)
Acetic acid	CH3COOH	CH3COO- (acetate)

Additional info: Hydronium ion is often written as H3O+ and is interchangeable with H+ in aqueous solutions.