Metalloids are generally not corrosive in their pure, elemental form, but this low reactivity changes dramatically when they form compounds. The stability of the pure element is a function of its specific atomic structure and bonding. Understanding the distinction between the elemental form and its compounds is key to assessing the risk they pose.
Defining Metalloids and Corrosiveness
Metalloids are elements located along the staircase line separating metals from nonmetals on the periodic table. They exhibit a blend of properties from both groups, often functioning as semiconductors that conduct electricity under specific conditions. The six most commonly recognized metalloids include Boron, Silicon, Germanium, Arsenic, Antimony, and Tellurium.
The term corrosiveness refers to the irreversible destruction of a material, typically a metal, through a chemical reaction with its environment. This reaction often involves oxidation and is most commonly seen in the form of rust or the action of strong acids dissolving a surface. For an element to be corrosive, it must be highly reactive and readily interact with other substances like water, oxygen, or biological tissue.
Elemental Reactivity
Metalloids in their pure, elemental state are largely unreactive under standard conditions, which is why they are not considered corrosive. This stability is due to their crystalline structure, which is often a hard, solid lattice with strong, stable covalent bonds between atoms. For instance, elemental Silicon exists as a hard, dark-gray solid with a diamond-like crystalline structure, making it chemically stable at ordinary temperatures.
The stability of elemental metalloids means that substantial energy or specific chemical reagents are needed to initiate a reaction. Unlike highly reactive metals that easily lose electrons, metalloids require extreme conditions, such as very high heat or specialized chemical environments, to react. Elemental Silicon does not react with water or most acids under normal conditions, though it is dissolved by hydrofluoric acid. Elemental Boron is similarly inert at room temperature, reacting only at high temperatures with most other nonmetals and metals.
The Role of Compounds
While the elemental forms of metalloids are chemically stable, their compounds are often highly reactive, toxic, or corrosive. Metalloids primarily form covalent bonds, and the resulting molecules can have properties that are vastly different from the parent element. This change in chemical structure and bonding determines the potential for danger.
Many inorganic compounds of metalloids are highly toxic, with Arsenic compounds being a primary example. Elemental arsenic is the least toxic form, but inorganic arsenic compounds, such as arsenic trioxide, are highly soluble and extremely poisonous. Arsenic trioxide is known for its ability to interfere with biological processes.
Some metalloid compounds exhibit extreme corrosive power, particularly those formed with halogens. For example, Silicon tetrafluoride is a volatile compound, and its precursor, hydrofluoric acid, is one of the few substances that can dissolve Silicon dioxide and silicates. This dissolving power allows the acid to etch glass and other silicate materials, making it a highly dangerous and corrosive agent.
Safety and Handling Considerations
The low reactivity of elemental metalloids does not translate to safety when handling the elements or their derivatives. Although pure metalloids pose little corrosive risk, their dust and fumes require careful handling due to potential toxicity. Industrial safety standards mandate strict controls, such as specialized ventilation and protective gear, when working with any form of these elements.
The greatest risk comes from metalloid compounds, which are widely used in manufacturing. Arsenic and Antimony compounds are highly toxic and used in applications like semiconductors and flame retardants. Therefore, while the pure solid element may sit inertly on a shelf, its chemical products must be managed under stringent safety protocols.