By Ron B. Davis Jr., Georgetown University
Aluminum is the least ‘weak’ of the p-block metals. It’s very light, yet very strong. Gallium, on the other hand, has one of the lowest melting points of all the metals on the periodic table. The p-block metals are all relatively soft metals with low melting points. Thus, these elements are sometimes depicted together under the names ‘weak metals’, ‘poor metals’, or ‘post-transition metals’.
At the atomic level, what sets the p-block metals apart from other metals is the greater number of protons in the nucleus and the presence of p-electrons in their valence shells. This makes them closer to an octet and less willing to release electrons than most other metals on the table. In other words, being a p-block metal means having shades of that covalent, nonmetallic character.
The p-block metals also have some similarities with the nearby zinc group and, to a lesser extent, even the copper group of elements.
Though unknown to ancient civilizations, aluminum is extremely common in the Earth’s crust—a fact that makes aluminum affordable today, now that we know where to find it and how to extract it.
The challenge, historically, was that although aluminum is a highly-reactive element whose small size gives its orbitals good overlap with oxygen and allows it to lock on to oxygen tightly, behaving in this respect like its metalloid neighbors, boron and silicon.
Aluminum’s relationship with oxygen is so strong that, unlike the other p-block metals, it is a devoted member of the ‘Team Silica’ set of lithophilic elements that form rocks in Earth’s crust. That said, aluminum is a special lithophile. Substitution of aluminum, with a valence of three, for silicon with a valence of four, in silica leaves one oxygen looking for another bonding partner.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Becoming a Precious Metal
So-called aluminosilicates need an additional metal ion to compensate for this substitution and therefore, often incorporate additional lithophilic metal ions that provide positive charges—like sodium, potassium, and calcium—which make up the balance in alumino-silicates. It makes possible a huge variety of rock-forming minerals called feldspars. Thus, if silicon and oxygen run the show on ‘Team Silica,’ aluminum’s position next to silicon in group 13 of the table makes it like a top recruiter—opening up a space for other metal ions to join in with the silica structure.
Like the alkali and alkaline earth metals that aluminum often recruits to rock-forming minerals, aluminum itself has a relatively high position on the activity series. This reactivity kept pure aluminum hidden from humankind until 1825.
The first metallic aluminum was obtained by reacting aluminum oxide with recently discovered potassium metal, which is even higher on the activity series. With the development of this method to obtain small amounts of aluminum, it quickly became a precious metal.
By the mid-1800s, Emperor Napoleon III of France was famously using dinnerware made of aluminum for himself and his most honored guests, leaving gold and silver utensils to guests of lesser status.
Uses of Aluminum
It was in the late 1880s that advances in the processing of the mineral bauxite, and improved electrolysis techniques, combined to make aluminum widely available to the masses. It is now used for far more than just impressing dignitaries. For example, aluminum is used in production of transmission lines for electrical power. Being a p-block metal, aluminum is actually not the best electrical conductor on the table. Copper is a better conductor that only offers half the electrical resistance of aluminum.
Nonetheless, cost and availability tilt in favor of aluminum, but there is yet another property that makes aluminum wire so attractive—lower density.
Aluminum’s atomic radius is fairly similar to many other metals, like copper, even though aluminum has an atomic mass significantly lower than copper. This makes an aluminum wire of the same size only one third as heavy as copper. Pure aluminum metal is also highly reactive, but we don’t normally notice this because aluminum quickly reacts with oxygen at its surface to coat itself with a thin passivating layer of tough aluminum oxide—in much the same way magnesium and beryllium do.
Located just below aluminum, by contrast, gallium takes the weak metal tit[le to an extreme. Gallium is a very special metal, as it has one of the lowest melting points of all the metals on the periodic table.
Having 31 protons compared to aluminum’s 13 is a huge jump in nuclear charge. Thus, gallium holds its valence electrons very tightly, making weaker metallic bonds than any other p-block metal. This is one of the factors that makes gallium’s melting point so low.
A Material of Choice in Alloys
And yet, gallium remains liquid over an incredible temperature range of almost 2200°! This is a wider liquid temperature range than that of any other element—or any other common material. This, combined with its low toxicity compared to other metallic liquids like mercury, has made gallium a material of choice in alloys.
One such alloy is Galinstan, which replaces mercury in high-temperature thermometers.
A New Class of Semiconductors
Gallium, can also be combined with nitrogen to form gallium nitride, a substance with properties similar to the intervening element between them, silicon. This new material may become the base material for a whole new class of semiconductors.
Other pairings of group 14 and 16 elements like this have also shown some promise in semiconductor applications that traditionally use pure group 15 elements, whether gallium-with-arsenic, indium-with-arsenic, or indium-with-antimony. One or more of these combinations may even prove to be tougher and longer-lasting than silicon or germanium-based technology.
In conclusion, one can say that what unites all these elements is that they possess many of the properties of metals—they are malleable and ductile, and they conduct electricity better than semi-conductors. In addition, the unique, weakly-metallic properties of the p-block metals not only makes most of them easy to come by, their unique properties also makes them valuable materials such as gallium in thermometers.
Common Questions about Gallium and Aluminum
At the atomic level, what sets the p-block metals apart from other metals is the greater number of protons in the nucleus and the presence of p-electrons in their valence shells.
It was in the late 1880s that advances in the processing of the mineral bauxite, and improved electrolysis techniques, combined to make aluminum widely available to the masses.
Gallium remains liquid over a temperature range of almost 2200°. This is a wider liquid- temperature range than that of any other element—or any other common material.