Bonding and Physical Properties

This lesson covers: 

  1. The 4 types of crystal lattice structure
  2. The physical properties of each crystal type
  3. Explaining the properties in terms of structure and bonding
  4. Predicting structure from physical properties

The 4 types of crystal lattice structure

There are 4 main types of crystal lattice structure:

  1. Ionic - e.g. NaCl
  2. Metallic - e.g. Mg
  3. Simple molecular - e.g. I2 and H2O.
  4. Giant covalent - e.g. diamond and graphite.


The table below illustrates the key structural features of the four main types of crystal lattice:

Table showing the four types of crystal lattice structure: ionic, metallic, simple covalent, and giant covalent, with corresponding diagrams and descriptions.

Physical properties of the four crystal types

IonicMetallicSimple molecularGiant covalent
BondingIonicMetallicCovalentCovalent
Melting & boiling pointsHighHighLowVery high
State at room temperatureSolidSolidUsually liquid or gasSolid
Electrical conductivityConductive when molten or in solutionHigh conductivityNon-conductiveNon-conductive (except graphite)
Solubility in waterSolubleInsolubleDepends on polarity of moleculeInsoluble

Ionic bonding explains ionic compound properties

High melting and boiling points:

  • Strong electrostatic forces between oppositely charged ions require significant energy to overcome.


Electrical conductivity:

  • When molten or dissolved, charged ions can move and conduct electricity.


Solubility in water:

  • Charged ions interact strongly with polar water molecules, allowing ionic compounds to dissolve.

Metallic bonding explains metal properties

High melting points:

  • Strong electrostatic forces between positive ions and delocalised electrons require significant energy to overcome.


Thermal conductivity:

  • Delocalised electrons readily conduct thermal energy, making metals good heat conductors.


Electrical conductivity:

  • Delocalised electrons can move and carry charge through metals.


Insolubility:

  • Metallic bonds are very strong, preventing dissolution.

Simple molecular bonding explains simple molecular properties

Low melting and boiling points:

  • Weak intermolecular forces between molecules require little energy to overcome.


Peculiar properties of ice and water due to hydrogen bonding:

  • Ice is less dense than water.
  • Water has a higher than expected boiling point.


Poor electrical conductivity:

  • Absence of charged particles or delocalised electrons prevents electrical conduction.


Solubility depends on molecular polarity:

  • Polar molecules (e.g., alcohols) can form hydrogen bonds with water and are soluble.
  • Non-polar molecules (e.g., oils) cannot form strong interactions with water and are insoluble.

Giant covalent bonding explains giant covalent properties

Very high melting and boiling points:

  • Many strong covalent bonds throughout the structure require significant energy to break.


Electrical conductivity:

  • Graphite: Delocalised electrons between carbon layers allow electrical conduction.
  • Diamond and silicon(IV) oxide: Localised electrons in four covalent bonds per atom prevent electrical conduction.


Insolubility:

  • Strong covalent bonds prevent interaction with water molecules.

Predicting structure from properties

You can use the properties of a material to predict its structure.

Example:

  1. Substance X has a melting point of 801°C. When solid, it is an insulator, but when molten it conducts electricity well.
  2. The high melting point and electrical conductivity when molten indicate substance X is ionic. It is not giant covalent because it conducts electricity when molten.


So substance X likely has an ionic crystal lattice structure.