Types of Substances

METALS

Metals have some characteristic properties:

malleability
ductility
electrical conductivity
thermal conductivity
relatively high melting/boiling points

This is due to the particles (cations in a sea of valence electrons) and bonding (strong, non-directional metallic bonds).

IONIC COMPOUNDS

Ionic Compounds have characteristic properties:

high/very high melting and boiling points
electrical insulators as solids
electrical conductors as liquids and solutions
hard and brittle
only soluble in polar solvents

Ionic compounds are made up of cations (positively-charged) and anions (negatively-charged). These particles are held together in a 3-dimensional lattice, with every ion having multiple bonds to neighbouring ions (of the opposite charge).

The particles (ions) are held together by very strong ionic bonds. Ionic bonds are directional electrostatic attracts between the oppositely-charged ions. This is what gives the bond its strength, and makes it inflexible.

COVALENT NETWORKS

Not all non-metals form molecules. Some create complex 3D networks, with the atoms bound together by multiple covalent bonds.

MOLECULES

We explore molecules in a lot more detail in the next part of learning. To be able to compare and contrast them with metals, ionic compounds and covalent networks, we do need some basic details about their structure and bonding:

We will explore why molecular substances have certain melting points, solubility etc. as we learn more about their intramolecular bonding and shape, as these determine the strength of the intermolecular bonds.

DISSOLVING

This is the hardest property to explain, especially for ionic compounds. The strength of the ionic bond suggests that ionic compounds should not dissolve at all. However, we know that salt (sodium chloride) dissolves in water. Why?

Water is a polar solvent. One side (pole) of water has a small positive charge. The other pole has a slight negative charge. The negative pole of multiple water molecules can be attracted to a cation, so surrounding it. The pull they have on the cation overcomes the ionic bond, so the cation dissolves.

At the same time, the same thing would happen with the positive pole of other water molecules being attracted to (and surrounding) an anion, so dissolving the anion.

Molecular compounds will dissolve if the attraction to the solvent is strong enough to overcome their attraction to other solute molecules. This is why polar solutes only dissolve well in polar solvents (like water).

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