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Chapter 2

Chapter 2. Elements, bonding, simple structures en ionic radii. Content. Bonding Metallic bonding Van der Waals b onding Ionic bonding Covalent bonding Ionic radii Radius ratio and coordination polyhedra Some general rules concerning the ionic structures. Introduction.

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Chapter 2

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  1. Chapter 2 Elements, bonding, simple structures en ionic radii

  2. Content • Bonding • Metallic bonding • Van der Waals bonding • Ionic bonding • Covalent bonding • Ionic radii • Radius ratio and coordination polyhedra • Some general rules concerning the ionic structures

  3. Introduction • As an introduction to chemical elements and bonding • Watch the video: The Formation of Minerals • Blackboard

  4. Metallic bonding Hexagonal close packing Cubic close packing (body centered) Cubic close packing (face centered)

  5. Metallic bonding • ‘Mobile’ electrons shared with positive atoms • Packing: Cubic close (fcc) or hexagonal close (hcp) or body centered cubic (bcc) • Cause highelectrical and thermal conductivity • Close packing of layers reason for: • Ductility, reflectivity, metallic lustre, optically opaque, malleable

  6. Van der Waals bonding Weak bonding, electrostatic attraction Low melting point, soft & compressible, low conductivity Not very important in minerals, except in mineral structures such as gibbsite, brucite and graphite

  7. Ionic bonding Electrostatic attraction Charge spread evenly, non-directional bonding – high symmetry NB in mineralogy; majority of minerals

  8. Covalent bonding Pairs of electrons shared by two or more atoms Depends on valence numbers Directional bonding – lower symmetry than ionic bonding Strongest bonding, insoluble, general med hardness, high melting point, low conductivity

  9. Bonding • Bonding is commonly a combination of more than one type of bonding. • Determine relative ionic vs covalent character (%) by Electronegativity difference more or less than 1.7

  10. Ionic radii

  11. Ionic radii Ideal square coordination ratio: 0.41 Ideal triangular coordination ratio: 0.15

  12. Radius ratio and coordination polyhedra

  13. Rules for ionic structures • Pauling’s rules: • Cation-anion distance determined by sum of ionic radii. Coordination number depends on radius ratio • Electrostatic charges should be balanced • When polyhedra share edges and faces the structure decreases in stability, due to cations coming into close proximity, especially for tetrahedra. So true ionic structures only shares corners or sometimes edges (octrahedral) If Pauling’s rules violated, the structure is not truly ionic but rather covalent

  14. Pauling’s rule no. 3

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