This blog was written by Dave Allen, an experienced IB Chemistry teacher. To read more Chemistry blogs for students and teachers, click here.

Knowing your bonds (and structures).

This blog post will (hopefully) refresh your memory on bonds and structures that you will need to familiar with in IB Chemistry.

Firstly, the bonds

When atoms join, we say they ‘bond’ and electrons are the part of the atom that are involved in bonding.

There are two types of bond that this blog post will cover:

  • Ionic
  • Covalent

All types of bond involve electrons arranging themselves so that the shells of each atom that has bonded are full – this is an important principle with bonding. No matter what type of bond forms, shells will always be full.

Ionic bonds involve transfer of electrons and are formed between metals and non-metals. Metals will lose electrons (they are oxidized) with the non-metals gaining the electrons (they are reduced).

Metals will form positive ions (sometimes referred to as cations) with non-metals forming anions (or negative ions).

The cations and anions are electrostatically attracted to each other, forming the ionic substance (an electrostatic attraction is an attraction between positive and negative charges).

Covalent bonds involve the sharing of electrons and are found between non- metal elements.

Each covalent bond is made of two electrons that are attracted to the nuclei of both the atoms in the bond. We often use ‘single’ bonds when two electron are shared (eg, the bond between two Hydrogen atoms), ‘double’ bond when four electrons are shared (the bond between two Oxygen atoms) and ‘triple’ bond when six electrons are shared (the bond between two Nitrogen atoms).

Ionic substances will form a giant lattice structure of repeating positive and negative ions. The attraction between the ions is strong so they tend to have high melting points. Ionic substances are usually soluble in water and will conduct electricity when molten or in a solution of water (aqueous) as the ions are free to move.

Covalent substances can form one of two structures, either a simple molecular structure or a giant covalent (macromolecular) structure. I think the easiest way of deciding what structure forms is to take the default position that it will be simple molecular unless the structure is that of diamond, graphite, silicon or silicon dioxide (these are the exceptions and form the giant molecular structure).

Simple molecular structures are made of small, discrete molecules. They have low melting points (due to the weak intermolecular forces between the molecules) and will not conduct electricity (as they do not form ions or contain delocalized electrons).

Giant macromolecular substances form a giant network of atoms that are bonded together via covalent bonds. Covalent bods are very strong and need a lot of energy to break so we find that these substances have very high melting points. Graphite is unusual as it can also conduct electricity as it has delocalized electrons that can move throughout the structure.

This blog post has only really scratched on the surface. When you go on and study this at IB level you will go into this in more detail and look at the structures but I hope this post will give you an idea of what to expect.

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