Re: why are the properties of Gallium considered unusual?

G'day Zoe.

Why are the properties of gallium considered unusual, and which are the unusual properties?
You have the description of some of the important properties of gallium just about right, although I would not have described it as 'volatile'. Why are some considered unusual? Well, they are unusual simply because they are the sorts of property you do not often find. Well over 99% of solids that melt are more dense than their liquid state. There is only one other solid I know of that floats on its melt -- water ice!

Gallium is one of about 65 elements that is metallic as a simple substance. It is a metal that is solid at room temperature, but would melt at body temperature. Among metals, only mercury melts at a lower temperature, and cesium is about the same. It has a wonderfully wide liquid range between its melting point around 30 deg C and its boiling point over 2000 deg C

That is about where it stops being unusual. In most other ways, it is a fairly typical metal. It is not particularly volatile -- mercury, zinc, and lead are much more so, to name just a few. It dissolves fairly readily both in acids and alkalis. Metals typically dissolve in acids, but a lot do not. Most do not dissolve in alkalis, but quite a few do -- zinc and aluminium, for example.

Gallium is less dense than copper or iron, but more dense than aluminium or magnesium, and similar to titanium. It is not unusual in that respect. Like all metals, it is a conductor of heat and electricity. It will alloy fairly well with a number of other metals.

Why does gallium have these unusual properties?
Crystalline solids arise from regular fixed arrangements of atoms or molecules. In the case of metals, there are no molecules as such involved. We are simply looking at an arrangement of atoms. All metals but a handful adopt one of three arrangements, where the atoms pack efficiently together like oranges in a fruit box. The three arrangements are known as 'cubic close-packed', 'hexagonal close-packed', and 'body-centred cubic'. Gallium is one of the small number of exceptions. The crystal it forms is much less symmetrical, and has the atoms much less efficiently packed together. It is such an inefficient packing that when the crystal melts, and the atoms start jostling around at random in the liquid, they actually pack together more efficiently, and take up less space. That is why the liquid is more dense than the solid. The inefficient packing is probably also the reason why the solid does not form so readily, and you have to get to a much cooler temperature than you might expect before it will freeze.

But that is not really an answer to the why. The real question is why cannot the spherical atoms of gallium pack together efficiently like the spherical atoms of any other metal? The answer is very complicated, and I do not think scientsists really fully know the detail of why this is so.

Another metal with an unusual packing is manganese, but in this case it is quite an efficient packing, and both density and melting point of manganese are similar to those of other metals nearby in the periodic table.

What are considered 'metallic' properties?
Everyone is familiar with what a metal is. But there is no particular way of defining it, and there are a number of borderline cases. For well over a century, chemists have considered 'metallic' in terms of a 'profile' of a number of typical properties. Not all metals have all of these properties. Many substances that are not metals have one or more of them. Here is a rough list of physical properties:

1. Metals are good conductors of heat and electricity. They are cold to the touch; they carry an electric current with little resistance. Even a typical good electrolyte has a resistivity about a thousand times that of a typical metal. But selenium and silicon are non metals that conduct electricity as well as most metals (unless you go to enormous lengths to make them ultra-pure), and some ceramics are better heat conductors than most metals.
2. Metals are opaque and lustrous. But so are materials like graphite, silicon, iron sulfide (pyrite or marcasite) and even iodine.
3. Metals can be beaten into thin sheets or drawn into wires. 'Malleable' and 'ductile' are the specialized words that are used. But nylon can be both malleable and ductile, and metals like tungsten are extremely brittle and intractable.

Chemical properties are even worse. When you say that metals typically react with strong acids, you have to remember that we are talking about a class of substances that ranges from potassium (which explodes in contact with water) to platinum or iridium (which will slowly dissolve in concentrated aqua regia if you stew them up for a month)!