Matthew Wilkinson, of Kings High School, asks :-

What is the most common element in the world?

Tony Reay, a chemist at Otago University, responded.

Our world contains 94 different elements although only a few are present in the earth in any quantity. The earth is divided chemically into three sections, a central core which is principally composed of iron and some nickel and surrounded by the mantle, chiefly of magnesium, iron, silicon and oxygen.

The third layer is the surface layer or crust which is more complex chemically. The crust of the continents is made up principally of aluminium, silicon and oxygen, the main components of silicate minerals which form most of the rocks of the surface of the earth. "Most common" depends on how the element is measured. An element like gold is more than 28 times heavier than an atom of lithium although in terms of volume they are about the same size. The commonest element in the earth's crust is oxygen which makes up about 47 per cent by mass, 63 per cent by the number of atoms, and since the atoms of oxygen are both big and of low mass, about 91 per cent by volume. Thus the rocks we see at the surface are mainly composed of large oxygen ions with the other elements filling in the gaps bewteen the oxygens.

The second commonest mineral by weight is silicon at about 28 per cent. Silicon and oxygen combine in the common mineral quartz, chemical formula SiO2. This mineral is hard and very resistant and survives weathering to form most of the white sand beaches found around New Zealand's coastline.