Classification of Elements and Compounds

Ionic Compounds

• Structure and Bonding: These substances form a giant ionic lattice held together by strong ionic bonds (electrostatic forces of attraction between oppositely charged ions).
• Melting and Boiling Points: They have very high melting and boiling points because significant thermal energy is required to overcome these strong forces. For example, sodium chloride ($NaCl$) melts at approximately $801^\circ\text{C}$.
• Electrical Conductivity: They do not conduct electricity when solid because the ions are locked into a fixed lattice. However, they are excellent conductors when molten (liquid) or dissolved in water (aqueous) because the lattice breaks down, meaning the ions are free to move and carry an electrical charge.
• Solubility: Most ionic compounds, such as magnesium sulfate ($MgSO_4$) and copper sulfate ($CuSO_4$), readily dissolve in water.

Simple Molecular Substances

• Structure and Bonding: These consist of small individual molecules (like $H_2O$ or $CO_2$). The atoms within the molecules are held together by strong covalent bonds, but there are only weak intermolecular forces existing between the separate molecules.
• Melting and Boiling Points: They have low melting and boiling points. When heated, only the weak intermolecular forces are broken, while the strong covalent bonds remain completely intact.
• Electrical Conductivity: They act as insulators in all states because they have no free ions or delocalised electrons to carry a charge.
• Solubility: Most are insoluble in water (such as hexane and liquid paraffin). Some polar molecules like sucrose (sugar) can dissolve in water, but even when dissolved, sucrose does not conduct electricity because no ions are formed.

Giant Covalent Structures

• Structure and Bonding: These are massive structures containing billions of atoms arranged in a regular repeating lattice, all connected by strong covalent bonds.
• Melting and Boiling Points: They have exceptionally high melting and boiling points because melting them requires breaking millions of strong covalent bonds.
• Electrical Conductivity: Most giant covalent structures, like diamond and silicon(IV) oxide ($SiO_2$), do not conduct electricity. Graphite is a notable exception; it conducts electricity because it has one delocalised electron per carbon atom.
• Solubility: They are completely insoluble in water and all other solvents.

Metallic Substances

• Structure and Bonding: Metals form a giant metallic lattice of positive metal ions surrounded by a "sea" of delocalised electrons, held together by strong metallic bonds.
• Melting and Boiling Points: They generally have high melting and boiling points due to the strong electrostatic attraction between the positive ions and negative electrons.
• Electrical Conductivity: They are excellent conductors in both solid and liquid states because the delocalised electrons are always free to move through the structure.
• Solubility: Metals are insoluble in water.

Comparing the Four Substance Types

By comparing how different substances respond to heat, water, and electricity, we can build a clear classification summary:

Substance Type	Typical Melting/Boiling Point	Solubility in Water	Conductivity (Solid)	Conductivity (Liquid/Aqueous)	Examples
Giant Ionic	High	Usually Soluble	No	Yes	Sodium chloride, Copper sulfate
Simple Molecular	Low	Usually Insoluble (some exceptions)	No	No	Hexane, Liquid paraffin, Sucrose
Giant Covalent	Very High	Insoluble	No	No (Graphite is an exception)	Silicon(IV) oxide, Diamond
Metallic	High	Insoluble	Yes	Yes	Iron, Copper

Step-by-Step Investigation of Unknown Substances

To determine the classification of an unknown substance in the laboratory, you can follow a sequential testing protocol:

1. Test Conductivity in the Solid State: Place the solid substance in an electrical circuit with a battery and a bulb. If it conducts, it is a metal (or graphite). If it does not, proceed to step 2.
2. Test Solubility: Add a small sample of the solid to a beaker of water and stir. If it dissolves, it is likely an ionic compound or a polar simple molecule.
3. Test Conductivity in Solution: If the substance dissolved in step 2, place electrodes into the solution. If the bulb lights up, it is ionic (because the ions are free to move). If it dissolved but does not conduct, it is a simple molecular substance.
4. Test Melting Point: Heat a dry sample of the substance using a Bunsen burner. If it melts very easily, it is simple molecular. If it does not melt under a standard flame, it is either an ionic compound or a giant covalent structure.