When beginning to learn the important concepts and knowledge required to engage with the DP Curriculum it is important to have a basic understanding and experience of some of the ideas in each topic. At AIS the majority of students follow the iGCSE Coordinated Science curriculum as a precursor to studying DP chemistry and this site will use some of the objectives from the iGCSE to provide an area of prior learning - allowing those students who would like to refresh their knowledge or for new students to engage wth matter that has become prior to starting this course.

To this end students are expected to have a basic understanding of the C3 Atoms, elements and Compounds with the direct aim of constructing formula for common elements and compounds and balancing symbol equations.

Simple Covalent Bonding - Molecules

So we have seen how Atoms can gain or lose electrons through IONIC Bonding and how Positive Ions can attract delocalised electrons in METALLIC Bonding - but can atoms SHARE electrons?

You should recall that Valency is the number of electrons in the outer shell of an atom. So Covalency is when Atoms outer shells join together to make full outer shells.

In the below diagram - you can see how many covalent bonds a typical atom of an element will form:

Here you can see that the Number of electrons an atom needs to fill it's outer shell is equal to the number of covalent bonds an atom will form.

If we use these rules then we can see that:

• Carbon always needs to have 4 covalent bonds
• Hydrogen always needs to have 1 covalent bond
• Oxygen always needs to have 2 covalent bonds
• Nitrogen always needs to have 3 covalent bonds

You cannot have less or more covalent bonds than this (using this model). Let's join Carbon and Hydrogen together:

• First draw the Outer (valence) shells fo Carbon and Hydrogen next to each other
• You should see that C has 4 electrons in it's outer shell and H has one.
• You can then OVERLAP the outer shell of the Hydrogen with the outer shell of the Carbon - place one electron of each INSIDE THE OVERLAP
• This overlap represents One Covalent Bond - as one electron comes from the Carbon and one electron comes from the Hydrogen we say they are SHARED
• The Hydrogen is complete as it has 2 electrons in it's outer shell (for H and He only) but Carbon now has 5 electrons - it needs three more!
• We can solve this by adding three more overlapping hydrogens as below: - you should also see that this model can be represented using stick models where each stick represents TWO Electrons
• Thus the molecule has the formula CH4

Another example can be seen below in water with the formula H2O:

In this example Oxygen - in Group 6 - only requires two covalent bonds to have a full shell and so forms two covalent bonds with Hydrogen - the rest of the electrons are placed in non-Bonding Pairs (called Lone Pairs).

All these atoms bond together to form Molecules with a relatively small number of bonds and atoms - these are defined as Molecules.

As molecules are so small it is relatively easy to break the small amounts of attractions between the molecules. This means they have Low boiling and melting points and are mostly gases at room temperature.

Properties of Molecules

Melting and Boiling Points/ State of Matter

Unlike Ionic and Metallic Substances - that have Giant Structures - Simple Molecules are relatively small with just a few atoms and bonds.

When simple molecules are melted or boiled - YOU DO NOT BREAK COVALENT BONDS - This is unlike Ionic and Metallic substances where their respective bonds must be broken in order to separate the particles.

Simple Molecules have WEAK FORCES OF ATTRACTION BETWEEN EACH OTHER CALLED INTERMOLECULAR FORCES.

These intermolecular Forces are very weak and only take small amounts of energy to break.

This means that Simple Molecules have LOW MELTING AND BOILING POINTS

AND THUS ARE OFTEN GASES AT ROOM TEMPERATURE.

It is also worth noting that Intermolecular Forces Increase with molecular size and so large molecules - with more bonds and atoms - have larger boiling points than smaller molecules (in general).

A WORD OF CAUTION ON INTERMOLECULAR FORCE - THERE ARE ACTUALLY 3 TYPES OF INTERMOLECULAR FORCE THAT WE STUDY IN DP CHEMISTRY - THIS MEANS THAT SOME MOLECULES MAY ACTUALLY BE LIQUIDS (LIKE WATER) OR EVEN SOLIDS (LIKE SUCROSE) AT ROOM TEMPERATURE

Electrical Conductivity

As there are no free moving ions or electrons Simple Molecules CANNOT CONDUCT ELECTRICITY IN ANY STATE