An element is composed of atoms that have the same atomic number. This means that each atom has the same number of protons in its nucleus as all other atoms of that element. Today 112 elements are known, of which 92 are known to occur in nature, while the remaining 20 have only been made with particle accelerators.

Oxygen is by far the most abundant element in the earth's crust at 46.6% and is almost half of the mass of the crust.

Silicon is a metalloid and is the 2nd most abundant element in the Earth's crust and accounts for 28% of the crust’s mass. A metalloid is a type of chemical element which has properties which lie in between those of metals and nonmetals.

Silicon when combined with oxygen forms silicate materials, such as silicon dioxide. Sand consists mostly of silicon dioxide, and quartz and other crystalline rocks are formed from other silicate materials. Silicon is also an essential material in the manufacture of electronics and computer chips.

Aluminium is the 3rd most common element in the Earth's crust with 8.1% of the crust’s mass. All the earth's aluminium is combined with other elements to form compounds, and it is never found as a lone element.

Iron is the 4th most common element in the Earth’s crust, accounting for over 5% of the crust’s mass and magnesium is the 8th most abundant element in the earth’s crust and comprises approximately 2.1% of the crust’s mass.

More Info: sciencing.com