Solid State Chemistry

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Solid State Chemistry (Fall 2016) Instructor: Jihoon Choi

8th week

193

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Solid State Chemistry (Fall 2016) Instructor: Jihoon Choi 194

1. Crystal Structures and Crystal Chemistry

2. Crystal Defects, Non-stoichiometry and Solid Solutions 3. Synthesis, Processing and Fabrication Methods

4. Characterization Technique 5. Bonding in Solids

Table of Contents

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Bonding in Solids

* What is Atom?

The atom is a basic unit of matter that consists of a dense central nucleus

surrounded by a cloud of negatively charged electrons.

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Bonding in Solids

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Bonding in Solids

* What is Molecule?

Under normal conditions no other element exists as an independent atom in nature, except nobel gases.

However, a group of atoms is found to exist together as one species

having characteristic properties. Such a group of atoms is called a

molecule.

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Bonding in Solids

* What is Chemical Bond?

The attractive force which holds various constituents (atoms, ions, etc.)

together in different chemical species is called a chemical bond

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Bonding in Solids

* Why Atoms combine to form molecule?

- Why do atoms combine?

- Why are only certain combinations possible?

- Why do some atoms combine while certain other do not?

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Bonding in Solids

Periodicity

Bond Type

Properties Structure

1) metal or non-metal 2) relative size

3) electronegativity difference

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Bonding in Solids

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* nebuliser

Because Pauling was an academic, he graded electronegativities on a 0 to 4.0 scale.

Electronegativity, symbol χ, is a chemical property that describes the tendency of an atom or a functional group to attract electrons (or electron density) towards itself.

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* Ionic, Covalent, Metallic, van der Waals and Hydrogen Bonding

- Sometimes more than one type of bonding is present

Li2SO4 : ionic bonds btw Li⁺ and SO42- ions and covalent bonds linking S and O within SO42- ions

TiO : ionic/metallic

CdI2 : ionic/covalent/van der Waals

- The bonding in a particular compound correlates fairly well with the position of the component atoms in the periodic table and , especially, with their electronegativity

- Most non-molecular materials have mixed ionic and covalent bonding and it is possible to assess the ionicity of a particular bond (the percentage of ionic character in the bond)

- The degree of covalent bonding increases with increasing valence and ions with a net charge greater than +1 or -1 are unlikely to exist

Bonding in Solids

NaCl = Na⁺Cl^- TiC = Ti⁴⁺C^4- ??? (non-ionic)

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* Ionic, Covalent, Metallic, van der Waals and Hydrogen Bonding

- An additional factor, especially in some transition metal compounds, in the occurrence of metallic bonding

- In metallic bonding, the outermost valence electrons are not localized on ions or in covalent bonds between atoms but are delocalized in energy bands which cover the entire structure - Van der Waals bonding is a feature of molecular materials in which strong covalent bonds hold the molecules together and weak van der Waals bonds are responsible for intermolecular interactions

- Hydrogen Bonds, such as between H2O molecules, are rather stronger since these involve ionic or polar interaction between partially charged H^δ+ and O^δ- atoms on adjacent molecules

Bonding in Solids

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Ionic, Covalent, Metallic, van der Waals and Hydrogen Bonding

Bonding in Solids

Ionic bonding

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* nebuliser

Ionic Bonding

Any characteristic features?

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* nebuliser

usual suspects

Ionic Bonding

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* nebuliser

Ionic Bonding

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* Ionic Bonding

- Purely ionic bonding rarely occurs, then why do we continue to use the ionic model?

; because of its apparent wide applicability and its usefulness as a starting point for describing structures which in reality often have considerable covalent bonding

- It leads to structures with high symmetry in which the coordination numbers are as high as possible, and the net electrostatic attractive force which holds crystals together is maximised.

* nebuliser

Ionic Bonding

* Ions and Ionic Radii

- Ions are essentially spherical

- A central core (with concentrated e- density) and an outer sphere (with very little e- density) - It is not obvious where one ion ends and another begins

- Instead of being incompressible, ions are fairly elastic

- This flexibility is necessary in order to explain variations of apparent ionic radii with coordination number and environment

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* nebuliser

Ionic Bonding

LiF

Li

F

* Instead of being incompressible, ions are fairly elastic