Types of Solids

Intermolecular Forces in Solids

The molecules in solids are held very tightly together, restricting their movement.

This means that the intermolecular forces are significant.

We can classify solids based on the type of intermolecular forces holding them together.

Types of Solids

We will classify solids into four types:

1. Molecular Solids2. Metallic Solids3. Ionic Solids4. Covalent network solids

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 -248oC) Solid CO2 (dry ice)

Solid H2O (ice) Iodine Sugar

Molecular solids

Consider the element Iodine, I2

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.

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 neighbouring atoms form a sort of ‘sea of electrons’

These electrons are called “delocalized electrons”

This ‘delocalized sea of electrons’ 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 sea’ that acts a glue that holds it together

The rice krispies are like the positive nuclei of the metal atoms that compose the solid.

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.

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 neat lookin’

Arrangement of Ions in a Sodium chloride cystal (cubic)

Covalent Network Solid

Consider carbon dioxide (CO2) and silicon dioxide (SiO2).

What would you expect the physical properties of SiO2 to be?

CO2 b.p. = -78.5oC If London forces are the only

intermolecular force, then you might predict the b.p. of SiO2 to be slightly more than CO2.

Covalent Network Solid

However SiO2 has a m.p. of 1650oC, and b.p. of 2230oC!

SiO2 is also known as quartz, or sand, is used in the production of glass.

Clearly, SiO2 is not a molecular solid like CO2 is.

SiO2 is a covalent network solid.

Raw silica (SiO2)

Silica glass (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, graphite, silicon, asbestos

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, it is a 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?

1. Silicon, Si2. Cesium, Cs3. Cesium iodide, CsI4. Ammonia, NH3

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

Homework

Read section 4.6 page 268 Page 273

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