Properties Of Aluminium: The Element
The properties of Aluminium are based on its atomic structure, as with all elements. Aluminium is of vital importance commercially, particularly to industries such as aviation and construction. Pure Aluminium is very reactive and so is not found in metallic form naturally but occurs in compounds such as Bauxite, from which it is mined and refined. There is more Aluminium present in the crust of the planet than any other metal.
Properties of Aluminium: Isotopes
There are only two naturally occurring isotopes of Aluminium, being 27- and 26-Aluminium. 27-Aluminium makes up virtually all of a given sample of Aluminium atoms; 26-Aluminium accounts for less than 0.1% of the element. This isotope is radioactive, leaving 27-Aluminium to be the only stable, naturally occurring form of the element. Hence the standard Aluminium atom is seen as having 13 protons and 14 neutrons.
Other isotopes have been identified. There are 22 in total, with numbers of neutrons in the nuclei ranging from 8 to 29. This corresponds to mass numbers of 21 to 42. With the exception of the two natural forms mentioned above, all these isotopes break down very quickly and are never found in nature.
Properties of Aluminium: Electron Structure
With 13 protons in the nucleus, the neutral Aluminium atom will possess 13 electrons. If we put 13 electrons through the electron details for subshell filling we get the configuration 1s2 2s2 2p6 3s2 3p1 which wives us THREE outer shell electrons, and therefore the electron dot diagram for this element as shown to the left. While this electron structure is still adequately represented by the Bohr diagram of the atom shown above, we need to visualize the layering of the electrons in the subshells. This is shown visually below. The distance of the subshell (shown by its line) indicates the relative energy level of that subshell; the further from the nucleus, the higher an energy level is represents.
Chemical Properties of Aluminium
The Aluminium nucleus has a much weaker pull on its outer shell electrons than does its vertical neighbour in the Periodic Table, the element Boron. This is due to the distance of the outer shell from the nucleus. Boron’s outer shell is closer to the nucleus and so while both Boron and Aluminium have 3 outer shell electrons, Aluminium’s hold on those electrons is much weaker. This is reflected in the difference in electronegativity values of these elements.
This difference accounts for the fact that Aluminium, virtually without exception, will shed its three outer shell electrons in order to achieve the stable state of a full outer shell. Once the three electrons in Aluminium’s third shell are gone, Aluminium has a full second (and now outer) shell. This results in the Aluminium atom becoming an ion with a positively charge of +3. This positive charge lets the Aluminium ion form ionic compounds with a massive range of other substances.
Properties of Aluminium: Why Doesn’t It Rust?
Aluminium is a reactive metal that willingly sheds outer shell electrons, so how is it that it stays in its metallic state once we have refined it? The answer lies in the way Aluminium metal rusts. When oxygen gas encounters the surface of the metal it absorbs electrons and oxide (O2-) ions form. These combine with the Al3+ ions from the metal to form Aluminium Oxide, Al2O3. This oxide forms a perfect seal over the surface of the metal and prevents any further oxygen from reaching the surface of the metal.
This accounts for the slightly dull appearance of Aluminium that we see in, say, a cooking pot. An interesting experiment is to take a knife and gouge the surface of the pot to reveal the shiny metal underneath the oxide, and then see how long it takes for that newly exposed surface to return to the dull appearance (ie, form a rust layer) as the rest of the pot. Aluminium products that are shiny, such as foil, are not pure Aluminium but are alloys that are not as prone to corrosion as the pure metal.