2.7 | ions and ionic compounds

Ionic Compounds and Ions

• Importance of Salt

  • Essential part of the human diet.

  • High value in regions where it is scarce.

  • Historical significance: Roman soldiers were partly paid with salt (Latin 'sal' → salary).

Predicting Ion Charges

• Understand how to predict the charge of ions formed by elements in groups 1, 2, 15, 16, and 17 of the periodic table.

• Ion Formation:

  • Ions are formed when electrons are added or removed from an atom, creating charged particles.

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

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

  • Example: Sodium (Na) has 11 protons and electrons. Losing one electron results in Na⁺ (net charge of +1).

  • Chlorine (Cl) gains one electron to become Cl⁻ (net charge of -1).

Writing Chemical Symbols for Ions

• Ions have a net charge indicated by a superscript. For example:

  • Na⁺ (cation)

  • Cl⁻ (anion)

• Polyatomic Ions:

  • Ions consisting of multiple atoms with a net charge, e.g., NH₄⁺ (ammonium), SO₄²⁻ (sulfate).

Chemical Properties of Ions

• The chemical properties of ions differ significantly from the atoms they derive from.

• Example: Sodium metal reacts violently with water; NaCl (sodium chloride) does not react violently.

Predicting Ionic Charges Using the Periodic Table

• Noble Gases:

  • Group 18 elements, chemically nonreactive, form very few compounds.

  • Atoms tend to gain or lose electrons to achieve electron configuration similar to noble gases (stable).

• Sodium loses one electron to emulate neon, chlorine gains an electron to resemble argon.

• Table 2.3:

  • Group 1: +1 ions

  • Group 2: +2 ions

  • Group 17: -1 ions

  • Group 16: -2 ions

Structure of Ionic Compounds

• Electron Transfer:

  • Example: Reaction between sodium and chlorine results in Na⁺ and Cl⁻ ions through electron transfer.

• Definition of Ionic Compounds:

  • Compounds made up of cations and anions; typically formed by metals and nonmetals (e.g., NaCl).

• Ions are arranged in three-dimensional structures. Empirical formula represents the simplest whole-number ratio of ions, e.g., NaCl (1:1).

• Charge Balancing in Ionic Compounds:

  • Overall charge must be neutral; total positive charge = total negative charge.

  • Example: Ba²⁺ combined with 2 Cl⁻ forms BaCl₂.

Elements Essential to Life

• Major Elements:

  • More than 97% of organic mass consists of six elements:

    • Oxygen (O)

    • Carbon (C)

    • Hydrogen (H)

    • Nitrogen (N)

    • Phosphorus (P)

    • Sulfur (S)

• Water:

  • Compounds most common in living organisms (70% of cell mass).

• Other Essential Elements:

  • 23 additional elements found in living organisms.

  • Five critical ions: Ca²⁺, Cl⁻, Mg²⁺, K⁺, Na⁺ (important for bone formation and nervous system functions).

  • Trace Elements:

    • Required in very small quantities (e.g., copper for hemoglobin synthesis in humans).

Conclusion

• Ionic compounds play a crucial role in both chemistry and biological systems, with their formation and structure dictated by the behavior of ions.