Crystalline Structures

Phase Changes Part 2

Solids: Crystal Structure

The crystal structure of a solid tells you how the solid's atoms are arranged. In order to understand melting, you need to understand crystal structure.

In diamond, carbon atoms are locked more tightly together than in any other substance. This is why diamond is the hardest natural substance on Earth.

Crystalline Solids and Amorphous Solids

Not all solids are crystalline.

A solid that has a regular, repeating crystal structure is called a crystalline solid. These include diamonds, graphite, and many other solids.

A solid that has no overall repeating pattern in its structure is called an amorphous solid.

Melting Point

The melting point of a solid depends on its crystal structure.

One of the properties of a solid is its melting point. The melting point is the temperature at which the solid changes phase into a liquid.

The crystal structure of a solid is a major factor in the melting point of crystalline solids.

Multiple Choice

What pattern do you see in the results?

• Solids made of molecules have the highest melting points.

• Solids made of metals have the lowest melting points.

• Solids made of molecules have the lowest melting points.

Melting Point Experiment Results

The structures of these solids have different basic units. Diamond is made out of only carbon (C) atoms, bonded together in a tight structure. Silver (Ag) is a metallic atom. Salt is made of ions, which are charged atoms. Ice is made of molecules, which are atoms held together by covalent bonds. 

Determining the Basic Unit

Determine the electronegativity difference between atoms in a solid in order to determine the basic unit of the solid.

To determine what type of basic unit (atoms, ions, molecules) is in a solid, you need to use the periodic table to determine if there will be a covalent or ionic bond between the different atoms.

4 Main types of solids

• Molecular solid

• Ionic Solid

• Network Solid

• Metallic solid

Network Solids and Metallic Solids

Network solids:

1) High melting point

2) Made of covalent bonds in specific directional structures

3) Very bad at conduction

Metallic solids

1) High Melting Point

2) Made of metallic bonds sharing electrons

3) Very GOOD conductors

Multiple Choice

Which is a better conductor?

• Metallic Solids

• Network solids

• Both are bad conductors

Network/metallic

Diamond has a network solid structure due to the atoms being covalently bonded in specific positions and act like one giant molecule. Silver consists of a metallic solid structure; all of the atoms share electrons.

The network structure of diamond makes the substance extremely difficult to melt. Transition metals, such as silver, tend to have fairly high melting points. 

Ionic Solids and Molecular Solids

Ionic Solids:

1) High Melting Point

2) Force: Ionic bonds

3) Very Bad conductivity

Molecular solids:

1) Low meting point depending on the IMF

2) Force: Intermolecular forces

3) Very Bad conductivity

Multiple Choice

Which is the best conductor?

• Ionic Solids

• Molecular Solids

• Network Solids

• Metallic Solids

Conductivity

Remember that electricity measures electron movement. 

Metallic solids are good conductors because their electrons are shared between many metal atoms, and the electrons can move freely from atom to atom.

Electrical Conductivity

Melting point is one important property of a solid. Another important property of solids is whether they conduct electricity.

Superconductors

Superconductors easily transmit electricity.

A superconductor has the ability to conduct electricity beyond that of a regular conductor.

When a conductor is cooled and starts to become a superconductor, it forms a magnetic field around itself. This field can even cause a magnet to float above the superconductor!

Semiconductors

Semiconductors have partial electrical conductivity.

The most important semiconductor is silicon. Most electronics use microchips made of solid silicon.

Semiconductors contain alternating layers of different atoms. Engineers can build semiconductor crystals one atomic layer at a time, and can manipulate the thickness of these layers at a microscopic level.

Thermal Conductivity

Thermal conductivity is the tendency to transmit heat.

Solids do not just conduct electricity — they also conduct heat. This property is called thermal conductivity.

Metallic solids, like a metal handle, are good thermal conductors for the same reason they are good electrical conductors. The free electrons in a metal transmit heat quickly through the metal. This is why a metal handle on a pot on the stove feels hot.

Wood is not a good thermal conductor. However, it is a good thermal insulator because it insulates against the flow of heat.

Breaking Solids

• Solids have different levels of hardness.

• Solids are also described by whether they break, bend, or scratch when you apply force to them.

• The hardness of a solid is determined by the bonds that hold it together. The stronger the forces, the harder the solid will be.

Multiple Choice

Which type of solid is softest?

• Ionic

• Network

• Molecular

• Metallic

Hardness of Solids

The hardness of a solid depends on intermolecular forces.

The forces within diamond are very strong. Graphite isn't nearly as hard as diamond because the forces between the layers in graphite are weak.

Malleability and Ductility

Malleability and ductility are two measures of hardness.

1) If a solid is malleable, it is bendable. A malleable solid, can be smashed down to a sheet.

2) Ductility is the ability to be stretched out without breaking. Something that is ductile, can be stretched into a wire. 

Brittle Solids

Brittle solids break instead of bending. If you apply a pressure to each of these objects, will they change shape or just break?

Most malleable materials are also ductile, but not always. Metallic solids tend to be both malleable and ductile.

Materials that are not malleable or ductile and break easily under an applied force are called brittle.

Alloys

Alloy is a mixture of two elements.

Steel is iron that has been mixed with carbon. This combination is called an alloy. An alloy is a solid that contains a mixture of two elements that has the properties of a metal.

Many common metals are alloys, such as steel, brass, and pewter.

Alloys are useful because the properties of elements can be combined.