Themes of the Day:

• Ions & Chemical Bonding
• Size of Atoms and Ions

Ions

• Ions - charged atoms with # of electrons not equal to # of protons - important for bonding
  • Anions - negative charge - # of Electrons > # of Protons - ex. Chlorine (Cl^-), Oxygen (O^2-)
  • Cations - positive charge - # of Electrons < # of Protons - ex. Sodium (Na⁺), Magnesium (Mg²⁺)
• Ionic charge = valence = oxidation state
• Periodic Table - helps predict ionic charges
• Electronegativity - Linus Pauling (1960) - propensity to gain or lose electrons - the difference in electronegativity between cation and anion can be related to the percent ionic character of the bond (more below)
• Ions with similar charges and sizes may substitute for each other in some minerals
• Abundance of elements in the Earth's crust:

  Element	Chemical Symbol and Common Ion(s)	Percent (by weight)
  Oxygen	O2-	46
  Silicon	Si4+	28
  Aluminum	Al3+	8
  Iron	Fe2+ or Fe3+	6
  Magnesium	Mg2+	4
  Calcium	Ca2+	2.4
  Potassium	K1+	2.3
  Sodium	Na1+	2.1
  All others		<1

Chemical Bonding

• Bonding is what happens when ions share electrons.
• Valence Related Bonding
  • Ionic bonds - electron(s) substantially transferred - e.g., Halite (NaCl) - tend to be brittle bonds
    • Balance of Forces
  • Covalent bonds - electron(s) more evenly shared - e.g., Diamond (C) - strong bonds (diamond is strongest)
    • sp hybrid orbitals in diamond
    • sp hybrid orbitals in graphite
    • more sp hybrid orbitals
  • Metallic bonds - transition metal nuclei swimming in a sea of shared electrons - e.g., Gold (Au) - malleable
    • valence and conduction bands
  • Range of Bond Character
  • Percent Ionic Character
• Bonds Not Involving Valence Electrons - generally weak (often feels greasy)
  • Hydrogen bonds - e.g., ice (H₂O) - Mohs Hardness = 3
    • Ice
  • van der Waals bonds - e.g., graphite (C) or talc (Mg₃AlSi₃O₁₀(OH)₂)
    • van der Waals bonds in graphite
• Silicate minerals, containing covalently bonded SiO₄ tetrahedra are most common in Earth's crust and mantle

Size of Atoms and Ions

• Effective Radius - also see Appendix A
• Measuring Bond Length
• Effective inoic radii as a function of charge
• Effective inoic radii as a function of coordination
• Spin state