Oxygen, Silicium, Aluminium, Iron and Calcium.

The choice to give some extra attention to these elements has to do with the fact of their abundancy; they are the most important elements in the outer earth crust of about 40 km thick. Humans have enormous profit from these elements.
About 20% of the air is Oxygen; it is number 2 after Nitrogen.
In the earth, Oxygen is the champion number one; about half of the substances in the earth is the element Oxygen, mainly in the form of oxydes.
The oxyde that occurs the most is Silicium oxyde, SiO2 (sand). This is used, for example, in glass and in very important applications of chips in computers and many other digital apparatus.
Aluminium, like many other elements, does not occur in pure form, but only in compounds.
Earth contains a high percentage of Al in the form of Bauxite, Aluminium ore, Aluminium oxyde. You must treat that in a proces of electrolysis to get the pure Aluminium (see module 10). It is a very strong en light metal.
Iron is known for a very long time and found in iron ore (Iron oxyde of course). They obtain the pure Iron in high furnaces with help of Coal.
Calcium is found in stony places, in particular in the form of calcium carbonate.