18.3 Structure and General Properties of the Metalloids

Structure and Properties of Metalloids

Structure and bonding of metalloids

Metalloids sit along the staircase line of the periodic table, and their atomic structures give them properties that fall between metals and nonmetals. This "in-between" character comes down to how their atoms bond and arrange themselves.

Boron (B) has an atomic number of 5 and sits in Group 13. Its electron configuration is $[He]\, 2s^2\, 2p^1$, which gives it only three valence electrons. That's fewer electrons than available orbitals, so boron is often described as "electron deficient." This deficiency drives it to form unusual covalent structures:

• Boranes are compounds of boron and hydrogen that use three-center, two-electron bonds to compensate for boron's electron shortage
• Boron clusters are polyhedral cage-like arrangements of boron atoms, stabilized by delocalized bonding across the cluster

Silicon (Si) has an atomic number of 14 and sits in Group 14. Its electron configuration is $[Ne]\, 3s^2\, 3p^2$, giving it four valence electrons. Those four electrons allow silicon to form four stable covalent bonds in a tetrahedral arrangement, much like carbon.

• In its elemental form, silicon adopts a diamond cubic crystal structure where each Si atom bonds to four neighbors in a rigid 3D network. This structure makes silicon hard and gives it a high melting point.
• Silicon forms a huge variety of compounds, including silicates (built from silicon and oxygen, often with other elements) and silicones (synthetic polymers with a repeating $Si-O$ backbone and organic side groups attached).

Chemical properties of boron vs. silicon

Boron compounds:

• Boron trihalides ($BX_3$, where X = F, Cl, Br, or I) are classic Lewis acids. Because boron has an empty p orbital, these molecules readily accept an electron pair from a donor molecule.
• Boric acid ($H_3BO_3$) is a weak acid. Rather than donating a proton directly, it acts as a Lewis acid by accepting a hydroxide ion from water. It can also function as a pH buffer in aqueous solutions.
• Boron nitride ($BN$) is isoelectronic with carbon, meaning each B-N pair has the same total number of electrons as a C-C pair. This lets BN form structures analogous to both graphite (hexagonal BN, a soft lubricant) and diamond (cubic BN, one of the hardest known materials).

Silicon compounds:

• Silicon dioxide ($SiO_2$) is found in sand, quartz, and glass. It's extremely stable and resistant to chemical attack, which is why quartz persists in nature while many other minerals weather away.
• Silicates make up the backbone of most rocks and minerals in Earth's crust. Their basic building block is the silicon-oxygen tetrahedron ($SiO_4^{4-}$), which can link together in chains, sheets, or 3D frameworks to produce the wide variety of silicate minerals you see in geology.
• Silicones are synthetic polymers with a $Si-O-Si$ backbone and organic groups (like methyl groups) attached to the silicon atoms. This structure gives them flexibility, heat resistance, and water repellency, making them useful in automotive, construction, and healthcare applications.

Both boron and silicon compounds tend to be less reactive than compounds of their neighboring elements. Their stable bonding arrangements mean that high temperatures or specific catalytic conditions are often needed to get them to react.

Industrial applications of silicon materials

Silicon is the second most abundant element in Earth's crust (after oxygen), which makes it cheap and widely available for industrial use.

Elemental silicon:

• Used as an alloying agent in steel and aluminum production to improve strength and durability
• When purified to very high levels (99.9999% pure), silicon becomes the semiconductor at the heart of modern electronics, including integrated circuits and solar cells

Silicon dioxide ($SiO_2$):

• The main component of glass, used in windows, windshields, bottles, and kitchenware
• Added as a filler in rubber and plastics to improve mechanical strength and lower costs
• In its porous form, silica gel works as a desiccant (the little packets you find in shoe boxes that absorb moisture)

Silicates:

• Essential for producing ceramics, cement, and concrete in the construction industry
• Zeolites, a class of porous silicates, serve as catalysts in petroleum refining and as molecular sieves that can separate molecules by size

Silicones:

• Used in adhesives, coatings, and sealants because they bond well and resist moisture and UV radiation
• Found in personal care products like shampoos and cosmetics for their smooth feel and water repellency
• Used for medical implants and prosthetics because they're biocompatible and resist degradation by bodily fluids

Metalloid characteristics and properties

A few general trends tie the metalloids together:

• Atomic radius generally decreases moving left to right across a period, just as it does for other elements. Smaller atomic radii tend to correlate with higher ionization energies and more nonmetallic behavior.
• Allotropy is common among metalloids. Silicon, for example, can exist in both crystalline and amorphous forms, each with different physical properties.
• Metallic vs. nonmetallic character varies across the metalloids. Elements closer to the metal side of the staircase (like germanium) behave more like metals, while those closer to the nonmetal side (like arsenic in some classifications) lean more nonmetallic.
• The band gap is what makes metalloids so useful. Metals have no band gap (electrons flow freely), and insulators have a large one (electrons can't flow). Metalloids have a small, intermediate band gap, which is why they're semiconductors: their conductivity falls between that of metals and insulators.
• Doping is the deliberate addition of small amounts of impurity atoms into a semiconductor crystal. Adding atoms with extra valence electrons (like phosphorus into silicon) creates an n-type semiconductor with more free electrons. Adding atoms with fewer valence electrons (like boron into silicon) creates a p-type semiconductor with "holes" that act as positive charge carriers. Doping is the foundation of how transistors and diodes work.