Types of Solids

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

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

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

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.

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.

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.

Gallium

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.

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

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

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

Arrangement of Ions in a Sodium chloride cystal (cubic)

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

Raw silica (SiO2)

Amethyst quartz (SiO2)

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)

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.

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.

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

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

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

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