Physical Properties

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Solid Structures

There are four key types of solid structures:

Formed by compounds of metals with non-metals, any salt is a good example of this structure.
- E.g. Sodium chloride (NaCl).
- E.g. Magnesium chloride (MgCl₂).
- E..g Lithium bromide (LiBr).

This form of crystal is seen for pure metals.
- E.g. Magnesium.
- E.g. Sodium.
- E.g. Aluminium.

One of the two possible structures formed by covalent bonding.
- E.g. Diamond.
- E.g. Graphite.

One of the two possible structures formed by covalent bonding.
- E.g. Ice.
- E.g. Iodine.

The properties of solids depend on their crystal structures.

Ionic crystals have strong electrostatic attractions between the ions.
This leads to high melting points.
The ions are fixed in place, so ionic crystals cannot conduct electricity as a solid.
The ions are charged, and so are soluble in polar solvents like water.
- In solution, the ions are mobile and can act as charge carriers. In solution, they will conduct electricity.

Giant covalent crystals have strong covalent bonds throughout the entire structure.
This leads to high melting and boiling points
Note: many giant covalent crystals will sublime (skip melting and go straight to a gas)
Giant covalent structures do not dissolve in anything.

Metals consist of a sea of free electrons surrounding positive ion cores.
The strong electrostatic attractions mean metals have high melting points.
- The melting point of a metal depends on the number of free electrons it donates to the sea.
The free electrons allow metals to conduct electricity as a solid.
- Metals can also conduct electricity as a liquid.
Metals do not generally dissolve in water. This is because many will react with water to form metal hydroxides.

Molecular compounds have very weak intermolecular forces.
They have low melting points and are often liquids or gases at room temperature.
Iodine is a solid at room temperature, but chlorine is a gas, for example.
They do not conduct electricity.
Some molecular solids may dissolve in water, but many will not. This is discussed later in the session on polarity.

Molecular solids and covalent structures are very different. This is reflected in their properties.

Molecular solids are held together by intermolecular forces.
- Intermolecular forces are forces between molecules that are NOT bonds.
These forces are fairly weak and easy to break.
- In melting or boiling a molecular solid, the intermolecular forces are broken, but the strong covalent bonds are not.

In giant covalent solids, the entire solid is one molecule.
The only forces present are intramolecular forces.
- Note the difference between intermolecular (between) and intramolecular (within) forces.
These forces are otherwise known as covalent bonds.
Intramolecular forces are much stronger than intermolecular forces.
- This means that giant covalent structures have much higher melting points because covalent bonds must be broken.