13. Aluminium

Aluminium (or Aluminum) has interesting chemistry and bears similarities to several different elements. It lies in Group III beneath boron, and with a 3p1 electron configuration, it is similar to boron’s 2p1 configuration. This implies that it will form a triangular (sp2 trigonal planar) geometry. It also has similarities to beryllium and can form bonds of a covalent nature. However, since aluminium is larger than boron, its valence electrons fill a larger volume and are therefore less coherent as a single quantum system. This makes aluminium more metallic in nature than the semi-metallic boron above it. (The wireframe indicates the boundary of the n=3 shell.)

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The small spheres above simply indicate the directions of maximum electron density. The orbitals themselves will be more like three longitudinal sections that can only occupy volume within their shell. The entire shell will be filled with electron density. It will be highest at the center of the face of each orbital (as in the traditional sp3 lobe shapes) and will decrease toward the nodal regions between orbitals, where electron density will be lowest (though not necessarily zero).

Aluminium’s three 3sp2-orbitals surrounding a full 2nd shell core.

Each of these three hybrid orbitals contains one electron. Like boron’s configuration, this arrangement is symmetrical in the equatorial plane but does not have equivalent symmetry in all directions. Aluminum readily makes three covalent bonds in order to pair up its three valence electrons, as we see in molecules like aluminum nitride (AlN) or aluminum oxide (Al2O3).

Aluminium is also willing to lose its three valence electrons in an ionic interaction, and it will lose all three in order to reach the stability of a full 2nd shell. This is the same electron configuration as the 2s22p6 noble gas configuration of neon — a multi-di-electron state with two concentric full shells. That is why aluminium forms a 3+ ionic state. The positive ion is much smaller than the neutral atom because protons now outnumber electrons by three. This results in a much higher effective nuclear charge — a higher average attraction by the nucleus on each electron, which shrinks the size of the electron shell as it is attracted inward with more force.

Neutral aluminium (Al) atom (L) compared to the much smaller Al3+ ion (R)

When we compare aluminium to boron, we see a significant size difference (see below). We also see a significant difference between the magnetic susceptibility of aluminium and the other Group III elements, since aluminium is paramagnetic (χm=16.5) while the others are diamagnetic. Aluminium’s paramagnetic strength is also very similar to that of sodium (χm=16).

Size comparison between boron (left) and aluminium (right)

This might support a conjecture that, in its crystalline metallic state, aluminium might delocalize only one of its three valence electrons, perhaps allowing the remaining two 3s-electrons to retain some of their preferred spherical di-electron character, stabilizing the 3rd shell with spherical electron density. If this conjecture is correct, it might account for aluminium’s interatomic distances being larger than expected, as well as the softness of the metal. It seems reasonable that the presence of a larger 1+ metallic core with a single delocalized electron per atom should result in weaker metallic bonds than a smaller 3+ metallic core with three delocalized electrons per atom.

Boron’s diamagmetism (χm=–6.7), on the other hand, would appear to emerge from the fact that it usually makes covalent bonds, in which all of its electrons are paired.

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