Types of solids

solids in chemistry

When we talk about solids we mean compounds, as all compounds can be solids at the appropriate temperature - they can exist in different states of matter depending on the temperature. This is an important concept to grasp - think of H₂O:

It's state of matter at room temperature is a liquid
It boils into a gas at 100 °C
It freezes into a solid at 0 °C

All other compounds can also exist in these states - it is easier to think of them all as one state when comparing properties; in this case solids.

THERE ARE FOUR TYPES OF SOLIDS

What determines the type of solid a compound is?

Below is a summary of the various inter molecular forces .

To change states of matter energy must be involved - energy which is needed to overcome the attractive forces holding molecules together.

It is important to understand here that we are not breaking apart the bonds that make up the actual molecule, but in the 'bonds' that attract the molecules together - these are Inter Molecular Forces. Thinking of H₂O again, each molecule is made of 2 O-H covalent bonds, with the discrete H₂O being held together by inter molecular forces- in this case permanent forces, known as permanent dipoles. This makes H₂O polar. In addition water can hydrogen bond.

molecular solids

These solids are made of discrete molecules held together by weak inter molecular forces
They are the weakest solids with the lowest melting and boiling points due to the weak inter molecular forces holding the molecules together
Very little heat energy is required to break them apart, thus many are liquids or gases at room temperature
These solids have no free charged particles or electrons so cannot conduct electricity

They show solubility but only in the right solvent - polar molecules will dissolve in polar solvents; the same is generally true for non polar molecules. Do not write like dissolves in like - it has to have a meaning!
Molecular solids can be made of the same atom, and are comprised of separate molecules, eg H₂O, CO₂, I₂, N₂, Cl₂
They are non metallic

metallic solids

Metals are all solid at room temperature, apart from Hg, due to the strong metallic bonds holding them together - they need large amounts of heat energy to overcome these bonds
Metals can be defined as a lattice of positively charged nucei (cation) held together by a sea of delocalised electrons - this means the attractive force between the cation and the electrons is multi-directional, which is important for metallic properties
The cations can be described as atoms without the valence electrons

Metallic properties: conductivity of electricity and malleability

Metals can conduct due to their free electrons, which are 'free' to carry electrical charge.

Metals are malleable and ductile due to their non directional forces of attraction between the cations and the delocalised electrons

ionic solids

Ionic solids are composed of a cation (positive metal ion) and an anion (negative non metal ion)
They are held together by the strong electrostatic attraction between the oppositely charged ions in a regular 3d lattice
Due to the strong attractive forces they have high melting and boiling points

PROPERTIES OF IONIC SOLIDS: BRITTLENESS, SOLUBILITY

Ionic solids are brittle as the same charged ions can align and repel each other
Ionic compounds generally dissolve in polar solvents, such as water

Ionic compounds dissolve in water to form aqueous solutions - water is a polar solvent and ionic compounds are at the extreme end of the covalent - polar - ionic bond continuum

PROPERTIES OF IONIC SOLIDS: CONDUCTIVITY

Ionic compounds can conduct electricity - this is due to the IONS / CHARGED PARTICLES and not free electrons -

it is an important distinction!

The animation shows what happens to the ions when electricity is passed through an aqueous solution:

*The cations migrate to the negative cathode

*The anions migrate to the positive anode

*In this way an electric current is conducted

Examples include NaCl, MgCl₂, LiF, CaCl₂, Na₂O, KBr, KI.

giant covalent solids

*Giant covalent network are solids made of atoms in an extended network held together by covalent bonds

*Covalent bonds take a lot of energy to break apart, hence these solids have high melting points and boiling points

GRAPHITE

*Graphite is only the covalent network solid that conducts electricity - this is due to its' structure which is a series of sheets of carbon atoms bonded to 3 others, with a delocalised electron between the sheets - these delocalised electrons allow electrical charge to be conducted

*The attractive forces between the sheets is weak allowing the layers to slide; hence graphite makes an excellent high temperature lubricant

*This also accounts for its softness

Most other covalent network solids are made of giant 3 dimensional structures in which all atoms are bonded to other atoms. This means:

*there are no free electrons so they cannot conduct

*they are rigid, hard and strong - diamond is the hardest naturally occurring substance measuring 10 mohs

*An exception is graphene - another 2 dimensional structure made of C atoms; unlike graphite there are no free electrons

TYPES OF SOLIDS QUESTION

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