1. Types of Solids SCH 4U1

2. Types of Solids  We will classify solids into four types: 1. Molecular Solids (Polar and NonPolar) 2. Metallic Solids 3. Ionic Solids 4. Covalent network solids (2D and 3D)

3. Molecular solids  A molecular solid is a solid that consists of atoms or molecules held together by intermolecular forces  Examples include:  Solid neon (melting point of -248 o C)  Solid CO 2 (dry ice)  Solid H 2 O (ice)  Iodine  Sugar

4. Molecular solids  Consider the element Iodine, I 2  Iodine is a solid at room temperature and pressure  What intermolecular forces maintain this solid state?  Only London forces!  However, it has 53 electrons, giving it sufficient intermolecular force to be a solid at room temperature.

5. Molecular solids  Molecular solids are held together more strongly if made up of polar molecules (dipole-dipole forces) rather than non-polar molecules (London dispersion forces).  Molecular solids tend to be soft, waxy, flexible, and have low melting points.

6. Metallic solids  Metals usually have very high melting points, although this can be highly variable.  For example, mercury melts at -39 o C while tungsten melts at 3410 o C  Therefore, the forces in metals must be quite strong.

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

9. Metallic solids  The positive nuclei of metals form lattice structures within the electron soup.  This means that metals are composed of crystals, but they are usually too small to see.  However, formation of metals through electroplating can make the crystalline nature more obvious.

10. 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.

11. Different Types of Crystal Lattices

12. Gallium

13. Ionic Solids  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 intra-ion force and results in ionic compounds having very high melting points.

16. 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 and when dissolved in water (electrolyte)  May dissolve in water, but not all ionic compounds do!

17. 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

18. Arrangement of Ions in a Sodium chloride crystal (cubic)

19. Covalent Network Solid  Consider carbon dioxide (CO 2 ) and silicon dioxide (SiO 2 ).  What would you expect the physical properties of SiO 2 to be?  CO 2 b.p. = -78.5 o C  If London forces are the only intermolecular force, then you might predict the b.p. of SiO 2 to be slightly more than CO 2.

20. Covalent Network Solid  However SiO 2 has a m.p. of 1650 o C, and b.p. of 2230 o C!  SiO 2 is also known as quartz, or sand, is used in the production of glass.  Clearly, SiO 2 is not a molecular solid like CO 2 is.  SiO 2 is a covalent network solid.

21. Raw silica (SiO 2 )

22. Silica glass (SiO 2 )

23. Amethyst quartz (SiO 2 )

24. Covalent Network Solid  A covalent network solid is a solid that consists of atoms held together in large networks or chains by covalent bonds. Very common with Group 14 elements - can make 4 bonds.  Every atom is covalently bonded forming a 3-dimensional network  Examples include: diamond, graphite, silicon, quartz, asbestos

25. Graphite - 2D network solid  It is a 2D covalent network solid, which means it is soft and shiny  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 is also an electrical conductor in one direction only because of delocalized electrons between sheets of carbon.

29. Diamond - 3D network solid  3D network solids are hard and are generally transparent or translucent  Diamond is different allotrope of carbon than graphite  Like graphite, it is a covalent network solid, but it is 3- dimensional lattice of carbon atoms.  This is what gives diamond its characteristic hardness.

32. Example Problem  Which of the four basic types of solids would you expect the following substances to be? 1. Silicon, Si 2. Cesium, Cs 3. Cesium iodide, CsI 4. Ammonia, NH 3

33. Answer 1. Silicon atoms might be expected to form covalent bonds with other silicon atoms. A covalent network solid would result. 2. Cesium is a metal; it is a metallic solid 3. Cesium iodide is an ionic substance; it exists as an ionic solid 4. Ammonia has a molecular structure, and is therefore a molecular solid