Let’s begin by discussing non-crystalline, or amorphous, substances:
Amorphous Substance
An amorphous substance is a solid whose particles have no orderly structure (random).
- they tend to fracture and lack well defined faces
- they exhibit a melting range due to random placement of particles within the sample
ex. rubber, quartz glass
Now that we know what crystals aren't, let's take a look at what crystals are:
Crystalline Substance
A crystalline substance is a solid whose particles are ordered in well-defined arrangements (a regular, repeating pattern).
- they cleave, producing flat faces at definite angles to each other
- they exhibit a melting point due to ordered placement of particles within the crystal
ex. diamond, table salt, quartz
Unit Cell
A unit cell is the smallest repeating unit of a crystal. For instance, sodium chloride unit cells contain one sodium ion and one chloride ion.
- stacking
together several unit cells produces a crystal lattice
- there are several types of unit cells, including primitive cubic, body centred cubic and face centred cubic
Crystalline Substances
There are four types of crystals – ionic, metallic, molecular and covalent network. We will start with the crystal type with which we are most familiar and move on from there.
Ionic Crystals
Ionic crystals consist of positive and negative ions held together by the electrostatic force of attraction within a crystal lattice.
Properties of Ionic Crystals
Mechanical Stress
- cleave under mechanical stress
- along the line of impact, the ions are shifted so that they repel, rather than attract each other and this forms a flat face and distinct angles
Electrical Conductivity
- there must be electrons or ions to carry the electrical energy, ionic substances do not conduct when solid (ions are “locked up” in the crystal), but do conduct when liquid or in solution (ions are free to move about)
- very high since the intermolecular ionic bonds are very strong and will take a lot of energy to break
- light can pass through without being affected (transparent) because the electrons are locked up in orbitals, so there is no interaction
- most
ionic solids dissolve in water since the ions are charged and can interact with
water’s dipole (see this link for a reminder of how ionic substances dissolve in water)
Metallic Crystals
Metallic crystals consist of metal atoms held together by metallic bonds.
A metallic bond is the electrostatic force of attraction between 2 ‘semi-positive’ metal ‘ions’ (nucleus and core electrons) and fast moving electrons that zip between the ions. These electrons belong to no single positive ion, but rather to the crystal as a whole (they are often referred to as delocalized electrons). This can happen because the ionization energy is low for metals, thus, the outermost electrons can move away from the parent atom easily. See the first illustration under Mechanical Stress for a visual.
Properties of Metallic Crystals
Mechanical Stress
- malleable and ductile
- even when the atoms are shifted out of position, the electrons maintain the forces of attraction
Mechanical Strength
- alloys – solution of two metals is harder than one metal (the added atoms lock the lattice into position and the atoms cannot slide past one another)
- small atoms (Li) – more brittle harder substance
- large atoms (K) – more malleable, softer substance since electrons in the "sea" are held more loosely
Melting Points
- depends on bond strength (Hg is low and W is high)
- increases with the number of valence electrons available for bonding (Na (1e) 97.5°C, Cr (6e) 1890 °C)
Electrical Conductivity
- excellent since mobile electrons move freely and conduct well
- very good since mobile electrons transmit kinetic energy rapidly
Interaction with Light
- shiny and opaque
- light energy falls on electron and electron vibrates easily and reemits the light with the same frequency and intensity
Molecular Crystals
Molecular crystals consist of atoms or molecules held together by weak intermolecular forces (dipole-dipole forces, H-bonding, London forces).
Properties of Molecular Crystals
Mechanical Strength
- brittle and it shatters, not cleaves (not enough attraction in the 2D plane to produce a flat face)
- very poor since there are no free electrons or ions to carry the electricity
Interaction with Light
- varies
Melting Point
- typically low since the there are weak intermolecular forces, which are easy to break
Homework # 27-30
