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Ionic Compounds: Structure, Formation, and Properties (GOB Chemistry Chapter 3 Study Notes)

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Ions

Introduction to Ions

Ions are charged particles that play essential roles in chemical and biological systems, including cellular signaling and transmission between nerve cells.

  • Ion Formation: Ions are formed when atoms gain or lose electrons.

  • Cations: Positively charged ions formed by the loss of electrons.

  • Anions: Negatively charged ions formed by the gain of electrons.

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

Ions and the Octet Rule

Electron Configuration and Stability

The octet rule states that main group elements tend to undergo reactions that leave them with eight valence electrons, achieving a stable noble gas configuration.

  • Alkali Metals (Group 1A): Lose one electron to form cations with a noble gas configuration.

  • Halogens (Group 7A): Gain one electron to form anions with a noble gas configuration.

  • Example: Magnesium (Mg) loses two electrons to form Mg2+ with a neon-like configuration.

Electron Configuration Example:

  • Neutral Mg:

  • Mg2+: (Neon configuration)

Ions of Some Common Elements

Predicting Ion Charges

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

  • Group 1A: Form +1 cations

  • Group 2A: Form +2 cations

  • Group 3A: Aluminum forms +3 cations

  • Group 6A: Oxygen and sulfur form -2 anions

  • Group 7A: Halogens form -1 anions

  • Transition Metals: Can form cations of multiple charges

Charge Prediction:

  • For metals: cation charge = group number

  • For nonmetals: anion charge = 8 - group number

Periodic Properties and Ion Formation

Trends in Ionization Energy and Electron Affinity

Periodic trends influence the tendency of elements to form cations or anions.

  • Ionization Energy: Energy required to remove an electron; lower for metals, higher for nonmetals.

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

  • Metals: Tend to lose electrons (form cations)

  • Nonmetals: Tend to gain electrons (form anions)

Naming Monoatomic Ions

Systematic Naming Conventions

Monoatomic ions are named according to specific rules based on their charge and element type.

  • Main Group Metal Cations: Name the metal followed by "ion" (e.g., potassium ion, magnesium ion).

  • Transition Metals: Use either the old (-ous, -ic) or new (Roman numeral) system to indicate charge.

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

Symbol

Old Name

New Name

Cr2+

Chromous

Chromium(II)

Cr3+

Chromic

Chromium(III)

Fe2+

Ferrous

Iron(II)

Fe3+

Ferric

Iron(III)

Cu+

Cuprous

Copper(I)

Cu2+

Cupric

Copper(II)

Polyatomic Ions

Structure and Naming

Polyatomic ions consist of two or more atoms covalently bonded, carrying a net charge due to an imbalance of electrons and protons.

  • Common Polyatomic Ions: Nitrate (NO3-), Sulfate (SO42-), Phosphate (PO43-), Ammonium (NH4+).

  • Example: The nitrate ion, NO3-, is composed of one nitrogen atom and three oxygen atoms.

Formula

Name

NO3-

Nitrate ion

SO42-

Sulfate ion

PO43-

Phosphate ion

NH4+

Ammonium ion

Ionic Bonds

Nature of Ionic Bonding

Ionic bonds are formed by the electrostatic attraction between oppositely charged ions, resulting from the transfer of electrons from metals to nonmetals.

  • Properties: Ionic compounds are typically crystalline solids with high melting points.

  • Example: Sodium chloride (NaCl) is formed by the transfer of an electron from sodium to chlorine.

Formulas of Ionic Compounds

Determining Chemical Formulas

Ionic compounds are electrically neutral; the total positive and negative charges must balance.

  • Rule: The number of cations and anions is chosen so that the net charge is zero.

  • Example: Ca2+ and Cl- combine to form CaCl2.

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

Naming Ionic Compounds

Systematic Naming of Compounds

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

  • Type I: Main group metal cations; charge does not vary (e.g., sodium chloride).

  • Type II: Transition metals; specify charge using Roman numerals (e.g., iron(III) chloride).

  • Do not: Use prefixes like di- or tri- for ionic compounds.

Chemical Name

Formula

Applications

Ammonium carbonate

(NH4)2CO3

Smelling salts

Calcium carbonate

CaCO3

Antacid, lime

Sodium bicarbonate

NaHCO3

Baking powder

Some Properties of Ionic Compounds

Physical and Chemical Properties

Ionic compounds exhibit characteristic physical properties due to the strong ionic bonds between ions.

  • High Melting and Boiling Points: Example: NaCl melts at 801°C.

  • Crystalline Structure: Ions are arranged in a repeating pattern.

  • Electrical Conductivity: Ionic compounds conduct electricity when dissolved in water.

  • Brittleness: Ionic solids shatter when struck sharply.

  • Solubility: Many ionic compounds are soluble in water.

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

Fundamental Ions in Acid-Base Chemistry

The hydrogen ion (H+) and hydroxide ion (OH-) are central to the concepts of acids and bases.

  • Acid: A substance that provides H+ ions in water.

  • Base: A substance that provides OH- ions in water.

  • Hydronium Ion:

  • Examples of Acids: Hydrochloric acid (HCl), sulfuric acid (H2SO4), phosphoric acid (H3PO4).

  • Examples of Bases: Sodium hydroxide (NaOH), barium hydroxide (Ba(OH)2).

Acid

Formula

Anion Derived

Hydrochloric acid

HCl

Cl- (chloride ion)

Sulfuric acid

H2SO4

SO42- (sulfate ion)

Phosphoric acid

H3PO4

PO43- (phosphate ion)

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