Chapter 2Elements, bonding, simple

structures en ionic radii

ContentBonding Metallic bonding Van der Waals bonding Ionic bonding Covalent bonding

Ionic radii

Radius ratio and coordination polyhedra

Some general rules concerning the ionic structures

Introduction

As an introduction to chemical elements and bonding Watch the video: The Formation of Minerals

Blackboard

Metallic bonding

Hexagonal close packing

Cubic close packing (body centered)

Cubic close packing

(face centered)

Metallic bonding

‘Mobile’ electrons shared with positive atoms

Packing: Cubic close (fcc) or hexagonal close (hcp) or body centered cubic (bcc)

Cause high electrical and thermal conductivity

Close packing of layers reason for: Ductility, reflectivity, metallic lustre, optically

opaque, malleable

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

Ionic bondingElectrostatic attraction

Charge spread evenly, non-directional bonding – high symmetry

NB in mineralogy; majority of minerals

Covalent bondingPairs 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

BondingBonding 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

Ionic radii

Ionic radii

Ideal square coordination ratio: 0.41

Ideal triangular coordination ratio: 0.15

Radius ratio and coordination polyhedra

Rules for ionic structures

Pauling’s rules:1. Cation-anion distance determined by sum of ionic

radii. Coordination number depends on radius ratio

2. Electrostatic charges should be balanced

3. 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

Pauling’s rule no. 3