HOW WAS THE EARTH FORMED? - BASIC INFORMATION ON THE FORMATION OF PLANET EARTH

The solar system formed about 4.6 billion years ago. The Earth and other terrestrial planets are believed to have formed by gathering together the so-called planetesimals.

Planetesimals are formed by the coalescence of fine- or coarse-grained mineral matters, metals, and gases of various kinds. As planetesimals stuck together mostly by gravity and the body thus formed grew larger, it became a precursor of terrestrial planet.

Some of these bodies were smashed by other bodies, and their fragments became meteorites. Hence, studies of meteorites would provide a lot of insight into the formation and the earlier state of the Earth.

The planet Earth was thus formed. Heat was created as the coalescence (of planetesimals) proceeded due to gravity, and heat also came from radioactivity of several radioactive elements such as aluminum-26. So the newly formed body was heated and the core was melted.

As the material becomes liquid (as a result of melting), the materials contained in the liquid separate out according to their densities. The more dense material would sink closer to the bottom (core).

Thus, the present layer structure of the Earth formed. The innermost core is a dense solid of about 1,200 km radius, whose density is about 12.6 g per cubic centimeter (12.6 ×106 kg/m3).

It is made of mostly iron metal and a small amount of nickel. By the way, the density of iron metal is only 7.8×106 kg/m3 under the ordinary pressure. The next layer is the outer core (up to 3,500 km from the center of the Earth), which is liquid and has a density of 9.5–12×106 kg/m3. The chemical composition seems to be about the same as that of the inner core.

There is an abrupt change in density in the next layer, mantle. The width of mantle is about 2,900 km (3,500–6,380 km from the center). Its density ranges from 4 to 5.5 ×106 kg/m3. The mantle is made of mostly magnesium–iron silicates (silicon oxides). The outermost layer is the thin crust of about 35–45 km on the land portion, and about 6 km under the ocean portion.

IRON (FE) CHEMICAL INFORMATION - THE CHEMISTRY OF IRON

What Is Iron? All you need to know about the element Iron (Fe)

Iron, a familiar metal, tends to rust, as everybody has seen. What happens chemically when iron rusts? Iron atoms in the metallic iron carries no electric charge Fe(0), in which 26 electrons (negatively charged) are orbiting around a nucleus that contains 26 protons (with positive charge) and 30 neutrons (with no electric charge).

This applies to an isotope 26Fe56, the most abundant isotope of element iron. But iron atom can lose its electrons. This process (loss of electrons) is called “oxidation.” Iron becomes either Fe(II) by losing two electrons or Fe(III) by losing three electrons (under normal conditions), though it can take Fe(I) (under special conditions).

Fe(0) is said to have been oxidized to Fe(II) or Fe(III). [Fe(II) means an iron atom that carries two positive charges; this is so because there are now only 24 electrons (negative charges), but there are still 26 positive charges at the nucleus.]

For this to happen you have to have a chemical entity that removes the electrons from the iron atom. Such an entity is called an oxidant or oxidizing agent. The iron atom is said to be a reductant or reducing agent in this process, for a chemical reaction in which a chemical entity (oxidant) gains electron(s) is called “reduction.”

Hence, oxidation and reduction reactions occur simultaneously and are like “head and tail” of a coin. Iron (Fe) reacts with oxygen in the air and is oxidized first to Fe(II) and then Fe(III) ending up with iron oxide Fe2O3. [Fe(II) can also be expressed as Fe2+ or FeII, and such a state is called an “oxidation state”; in this case, the oxidation state of iron atom is +2 or II. Likewise, Fe(III)=FeIII=Fe3+.

We will usually use Roman numerals to express the oxidation states in this book]. Here oxygen (O2) in the air is the oxidizing agent. In the process, oxygen O2 which is in “zero” oxidation state gains four electron and is reduced to two of O−II (−2 oxidation state); therefore, the chemical reaction is + → III ( −II ) 2 2 3 4Fe(0) 3O 2Fe O . In this chemical reaction, 12 electrons are exchanged between four iron atoms and three oxygen molecules.

When this is not purely oxide and contains hydroxide Fe(OH)O or Fe2(OH)2O2, it shows that rust color, brown. Pure oxide Fe2O3 forms an ore called “hematite,” which is red. The red bed is found in many geological locations.

These descriptions suggest that iron, when forming chemical compounds, takes the form of Fe(II) or Fe(III). And it can go back and forth between Fe(II) and Fe(III) readily. Fe(II) gives off an electron to become Fe(III), and Fe(III) becomes Fe(II) when it accepts an electron. This kind of process is also called “electron transfer” reaction.

Hence, iron (in the form of Fe(II) and Fe(III)) can readily undergo an “electron transfer” reaction or alternatively an “oxidation–reduction” reaction, because the process of Fe(II)s becoming Fe(III) is an oxidation and the reverse (Fe(III) → Fe(II)) is a reduction reaction.

Some of you might have experienced, as this author has, to have your toilet bowl and others stained brown by your well water. The water underground can contain (depending on the location and other conditions) iron compounds; the iron is in the form of Fe(II), which is dissolved in water and almost colorless. It remains as Fe(II) in the underground, because no oxidant such as oxygen in the air is available.

However, once pumped out above ground and being exposed to the air, the iron soon turns into Fe(III) (through oxidation by oxygen). Fe(III) in water (neutral water, that is) is not stable, and soon reacts with water itself and forms iron hydroxide Fe(OH)3, which is brown and precipitates.

This is the brown stain. And the fact of the easy formation of iron stain suggests an easy affinity or reaction of Fe(II) with oxygen O2. This is indeed the basis of the usefulness of iron in the biological systems and our health.