Compounds form when atoms of different elements combine or join together to form particles of a new substance, during a chemical reaction. The particles of the new substance have different properties to those of the starting particle, but the actual atoms remain the same. They are just joined to different atoms, in a different order.

For example sodium reacts with chlorine to produce sodium chloride.

INSERT [NOT_CI_04_vid] 

What did you obseve in the video clip?

At the start sodium is seen as a silvery solid and chlorine is seen as a very pale green gas. At the end sodium chloride is seen as a white 'mist' ie a very fine white powder is seen throughout the whole gas jar.

How do we know particles of different substances are involved in this reaction?

Althought we can not see what the actual particles look like (on a sub-microscale), we can observe that the bulk properties of the two substances at the start of the reaction are different to those observed at the end of the reaction. Once the gas jar full of chlorine was put over the hot sodium, nothing entered or left the system. Therefore we can conclude that the white 'mist' must be a result of particles of different substances reacting together to form particles of a new substance..

Chemists use symbols and formulae to represent the particles involved in a chemical reaction and chemical equations to tell the story of what is happening during the reaction.

You can learn more about elemental symbols, chemical formulae and equations by accessing the quantitaitve chemistry course.

What is the equation for the reaction between sodium and chlorine?

Sodium + chlorine → sodium chloride

2Na(s) + Cl₂ (g) ↓ 2NaCl (s)

Later on in this course, we will be looking at how different particles combine in much more detail.