1. Types of Solids

2. Intermolecular Forces in Solids
The molecules in solids are held very tightly together – this shows HIGH intermolecular forces
Intermolecular forces = between molecules/particles
Intramolecular forces = between atoms in a molecule/compound
We classify solids based on intermolecular and intramolecular forces

3. Types of Solids We will classify solids into four types:
Molecular Solids
Metallic Solids
Ionic Solids
Covalent network solids

4. Molecular solids
A molecular solid one type of covalent solid (2+ nonmetals)
Examples include:
Solid neon (melting point of -248oC)
Solid CO2 (dry ice)
Solid H2O (ice)
Iodine
Sugar

6. Metallic solids
Metals usually have very high melting points, although this can be highly variable.
For example, mercury melts at -39oC while tungsten melts at 3410oC
Therefore, the intermolecular forces in metals must be rather strong.
These forces are not completely understood.

7. Metallic solids
In metals, the valence electrons of neighboring atoms form a sort of ‘electron sea’
These electrons are called “delocalized electrons”
This ‘delocalized electron sea’ can be thought of as a ‘glue’ that holds the positive nuclei of the metal atoms together.

8. Metal solids can be thought of as being like a rice krispy square
The marshmallow is like the ‘delocalized electron soup’ that acts a glue that holds it together
The rice krispies are like the positive nuclei of the metal atoms that compose the solid.

9. Gallium

10. Ionic Solids Consider sodium chloride (table salt)
An ionic solid is a solid that consists of cations and anions held together by the electrical attraction of opposite charges (ionic bonds)
This is technically considered an intramolecular force.

12. Ionic Bonding Ionic compounds have the following physical properties:
Very high melting points
Brittle
Non-conductive in a solid state
Conductive in a liquid state

13. Ionic Bonding
The forces involved in ionic compounds are derived from the creation of a crystal lattice composed of alternating negative and positive ions.
Because of the full charge on ions, ionic bonds are stronger than dipole-dipole interactions

14. Different Types of Crystal Lattices
No, you don’t need to know these! They’re just cool looking 

15. Arrangement of Ions in a Sodium chloride cystal (cubic)

16. Covalent Network Solid
Consider carbon dioxide (CO2) and silicon dioxide (SiO2).
We might expect these compounds to be similar, but they are actually very different!
CO2 b.p. = -78.5oC
SiO2 b.p. = 2230oC
This is because SiO2 is a covalent network solid

17. Raw silica (SiO2)

18. Amethyst quartz (SiO2)

19. Covalent Network Solid
A covalent network solid is a solid that consists of atoms held together in large networks or chains by covalent bonds.
Every atom is covalently bonded forming a 3-dimensional network
Examples include: diamond (carbon), graphite (carbon), silicon (SiO2), silicates, mica, and fullerene (aka carbon nanotubes)

20. Graphite Graphite is an allotrope of carbon
It is a covalent network solid
It consists of ‘sheets’ of covalently bonded carbons
These sheets can slide over one another easily, making graphite useful for writing (i.e. it is the ‘lead’ in pencils!)
It also is also an electrical conductor.

22. Diamond Diamond is another allotrope of carbon.
Like graphite: covalent network solid.
However, instead of sheets, it forms a 3-dimensional lattice of carbon atoms.
This is what gives diamond its characteristic hardness.

24. Example Problem
Which of the four basic types of solids would you expect the following substances to be?
Silicon, Si
Cesium, Cs
Cesium iodide, CsI
Ammonia, NH3

25. Answer
Silicon atoms might be expected to form covalent bonds with other silicon atoms. A covalent network solid would result.
Cesium is a metal; it is a metallic solid
Cesium iodide is an ionic substance; it exists as an ionic solid
Ammonia has a molecular structure, and is therefore a molecular solid

26. Solids ranked by strength of forces:
1: Network covalent solids 2: Ionic solids 3: Metallic solids 4: Molecular solids

27. Organizing our Ideas: Summary Table
Create a table which summarizes the following characteristics for each of the four types of solids discussed:
Predominant force
Type(s) of elements included
Typical Melting point (low, med, high)
Malleability
Electrical conductivity