Types of Solids SCH 4U1

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)

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

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.

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.

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.

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.

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.

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.

Different Types of Crystal Lattices

Gallium

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.

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!

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

Arrangement of Ions in a Sodium chloride crystal (cubic)

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. = 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.

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.

Raw silica (SiO 2 )

Silica glass (SiO 2 )

Amethyst quartz (SiO 2 )

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

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.

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.

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

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