Fix the mistake: Matter and Chemical Bonds

Ionic Bonds and Ionic Crystals

A bond formed when a cation and an anion are joined by electrostatic force (Coulomb force) is called an ionic bond. Sodium chloride is a typical example.

Basic Knowledge

Metal elements readily become cations, while nonmetal elements readily become anions. When they meet, they exchange electrons and are joined by electrostatic attraction. This is an ionic bond.
Example: Na + Cl → Na+Cl- (= NaCl).
Substances with ionic bonds are called ionic crystals, in which cations and anions are arranged alternately in three dimensions. NaCl has an arrangement close to a face-centered cubic structure: six Cl- ions surround each Na+, and vice versa.
Empirical formula: Ionic substances are represented by the simplest whole-number ratio of cations to anions, as in NaCl. The ratio is determined so that the total charge is 0 (for example, Ca2+ and Cl- give CaCl2, and Al3+ and O2- give Al2O3).

Key Terms
Ionic bond（A bond caused by electrostatic attraction between cations and anions）
Ionic crystal（A crystal composed of ionic bonds, such as NaCl）
Empirical formula（The simplest whole-number ratio of cations to anions. NaCl, CaCl2）
Coulomb force（Electrostatic force between charges. It is inversely proportional to the square of distance）
Electrolyte（A substance that dissolves in water and conducts electricity. Common among ionic substances）
Ionic formula（A formula representing an ion. Na+, SO42-）

Further Study (Background and Significance)

Characteristics of ionic crystals:
① High melting points (801℃ for NaCl) because Coulomb forces are strong.
② They are hard but brittle. When force shifts the ions in one direction, repulsion causes them to break (cleavage).
③ Solids do not conduct electricity because ions cannot move, but aqueous solutions and molten substances conduct electricity because ions can move freely; they are electrolytes.
④ Many are soluble in water because polar water molecules pull ions apart and hydrate them.
Familiar examples:
・Table salt NaCl: seasoning and physiological saline
・Calcium carbonate CaCO3: limestone, shells, eggshells, coral
・Copper(II) sulfate CuSO4: blue crystals; desiccants and poison detection
・Sodium hydroxide NaOH: caustic soda; a strong base
・Ammonium chloride NH4Cl: an example of an ionic bond containing a polyatomic cation
When polyatomic ions are included, treat each whole ion as one unit when writing the empirical formula (for example, (NH4)2SO4).

Key Points
  • Ionic bond = electrostatic bond between cations and anions
  • Many consist of a metal element and a nonmetal element
  • Empirical formulas use the simplest whole-number ratio with total charge=0
  • NaCl is 1:1, CaCl2 is 1:2, and Al2O3 is 2:3
  • They have high melting points and are hard but brittle
  • Solids are insulators; aqueous solutions and liquids conduct electricity
  • Many dissolve readily in water (hydration)
  • Use parentheses for polyatomic ions: (NH4)2SO4

Points to Note (Common Confusions)

① Empirical formulas (NaCl) and molecular formulas (H2O) are different. Ionic crystals have no molecular units, so empirical formulas are used. ② The “formula mass” of NaCl is the atomic mass of Na plus the atomic mass of Cl; it is not called “molecular mass.” ③ The boundary between ionic bonds and covalent bonds is continuous according to the electronegativity difference. A completely ionic bond is an idealized concept. ④ Substances containing ammonium ions NH4+ have ionic bonds between nonmetals, such as NH4Cl.

Practice

  1. Write the empirical formula of the substance made from calcium ions Ca2+ and chloride ions Cl-.
  2. Explain why an ionic crystal does not conduct electricity as a solid but does conduct electricity in aqueous solution.
  3. Which is expected to have the higher melting point, NaCl or MgO? Include the reason.

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