Chemistry 2e · Representative Metals, Metalloids, and Nonmetals
Structure and General Properties of the Metalloids
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In 30 seconds
The metalloids — boron, silicon, germanium, arsenic, antimony, and tellurium — sit along the diagonal "staircase" between metals and nonmetals (with polonium and astatine sometimes included). They look like metals (shiny, brittle solids) but behave chemically like nonmetals, with intermediate electronegativity, ionization energy, and — the property that made them famous — conductivity that can be finely tuned by Doping Deliberately adding tiny amounts of impurity atoms Full entry →.
The structure explains the properties. Silicon and germanium crystallize in the Diamond cubic Structure where each atom bonds covalently to four neighbors Full entry → structure: each atom covalently bonds to four neighbors in a giant three-dimensional network, like carbon in diamond. Boron is different, forming a unique lattice of B₁₂ icosahedra — clusters of twelve boron atoms. Held together by directional covalent bonds rather than a delocalized "sea of electrons," these solids conduct far worse than metals, and electrons are not free to roam until the material is given energy (light, heat, or an electric field).
The defining chemical property is amphoterism — acting as either a metal (losing electrons) or a nonmetal (gaining or sharing them) depending on the reaction partner. Their oxides react as acids with bases and as bases with strong acids. This duality, plus tunable conductivity, makes metalloids the backbone of modern electronics: silicon chips, germanium detectors, gallium arsenide lasers, and antimony flame retardants.
Why this matters
- The digital revolution runs on silicon: integrated circuits are carved from purified, deliberately "doped" silicon. Understanding why silicon is a Semiconductor Material whose conductivity rises when energy is added Full entry → — and how trace impurities tune its conductivity — explains how transistors work.
- Optics and sensors: germanium serves in infrared optics and radiation detectors; tellurium appears in cadmium telluride solar cells; gallium arsenide (from arsenic) powers LEDs and high-speed chips.
- Everyday materials: borosilicate glass (Pyrex-style cookware, lab glass), boron fibers, antimony flame retardants, and semiconductor alloys all trace back to Metalloid An element with properties between metals and nonmetals Full entry → chemistry.
- Exams: expect the staircase, semiconductor behavior (conductivity rising with temperature, unlike metals), and Amphoteric Able to react as both an acid and a base Full entry → oxides.
The college version
Core Concepts
Where the metalloids live: the staircase
A diagonal band separates metals (left and below) from nonmetals (right and above) on the periodic table; the elements bordering it — B, Si, Ge, As, Sb, Te — are the metalloids. Their positions reflect a balance: ionization energy and electronegativity increase up and to the right, so neither metallic nor nonmetallic character wins decisively. Silicon's electronegativity (1.90, Pauling scale) lies between aluminum (1.61) and carbon (2.55).
Bonding and crystal structure
- Silicon and germanium: diamond cubic. Each atom forms four covalent bonds to neighbors, building a giant network — hence hard, brittle solids with high melting points (Si: 1414 °C; Ge: 938 °C).
- Boron: icosahedral clusters. Boron's electron deficiency (three valence electrons for four bonding sites) forces twelve atoms into B₁₂ icosahedra that then bond to each other. Boron is extremely hard (near diamond), melts near 2076 °C, and exists in several allotropes built from these clusters.
- Arsenic and antimony: layered structures with some metallic character, matching their lower staircase position.
- Allotropy: like carbon, several metalloids show multiple allotropes (amorphous vs. crystalline silicon; gray and yellow arsenic) whose properties differ strongly.
Semiconductors: the conductivity that changed the world
Metals conduct because valence electrons are delocalized; metalloids conduct poorly when cold because their electrons are locked in covalent bonds. The Band gap Energy gap between the filled valence band and empty conduction band Full entry → — the energy separation between the filled valence band and the empty conduction band — decides the behavior:
- Gap zero or tiny: electrons flow freely — metal.
- Gap large (several eV): electrons almost never jump — insulator.
- Metalloids like silicon (band gap ≈ 1.1 eV) and germanium (≈ 0.7 eV) sit in between: near absolute zero they are insulators, but at room temperature — or under light or an applied field — some electrons jump the gap and conduct. This defines a semiconductor: conductivity that rises with temperature and added energy, the opposite of a metal, where hotter atoms scatter electrons and conductivity falls.
Doping is the metalloid's superpower: replacing a tiny fraction of Si atoms with a group-13 atom (boron or gallium — one fewer valence electron) creates electron "holes" — p-type; a group-15 atom (phosphorus or arsenic — one extra electron) adds free electrons — n-type. The p–n junction is a diode — the basis of transistors, solar cells, and LEDs.
Chemical behavior: amphoteric oxides and covalent compounds
Metalloids form covalent compounds — their electronegativities are too high to give electrons away completely — and their oxides are amphoteric or weakly acidic:
- B2O3 dissolves in water to give boric acid, H3BO3, and reacts with bases to form borates.
- SiO2 (quartz, sand) is a giant covalent network solid and weakly acidic: it reacts with molten NaOH to form sodium silicate, SiO2 + 2NaOH → Na2SiO3 + H2O, and is attacked by HF — which is why HF must be stored in plastic, never glass.
- As2O3 and Sb2O3 are amphoteric, reacting with both strong acids and strong bases. As2O3 is notoriously toxic.
- Hydrides are covalent, volatile, and often hazardous: SiH4 (silane, pyrophoric), AsH3 (arsine, highly toxic), SbH3 (stibine) are handled only under strict controls.
Trends down the staircase
Down a group (C → Si → Ge → Sn → Pb) metallic character increases — germanium looks metallic; tin and lead are true metals. Across a period (Si → P → S → Cl) nonmetallic character increases. The metalloids are the transition zone of both trends.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Metalloid | Metal or nonmetal | Metalloids are intermediate: semiconducting, amphoteric oxides, covalent structures. |
| Semiconductor conductivity | Metal conductivity | Metals conduct better when cold; semiconductors conduct better when hot. |
| Doping with group 13 | Doping with group 15 | Group-13 (B, Ga) creates holes → p-type; group-15 (P, As) adds electrons → n-type. |
| SiO₂ as "acidic oxide" | SiO₂ dissolving in ordinary acids | SiO₂ reacts with HF and molten base, but is otherwise inert to acids — that's why HF etches glass. |
| Silicon (element) | Silicone (polymer) | Silicon is the metalloid element; silicone is a synthetic Si–O polymer in sealants and cookware. |
| Band gap in eV | Photon energy in J | Convert eV to J (× 1.602 × 10⁻¹⁹) before using E = hc/λ. |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Some elements are like metal (electricity zooms through) and some are like plastic (they block it). The metalloids are the in-between kids: they can block electricity sometimes and let it through sometimes, and scientists can even "tune" them by adding a few extra atoms. That tuning is how your phone's brain — a silicon chip — knows what to do.
Worked example
Example 1: Making silicon — stoichiometry of the carbothermic reduction
Silicon is produced industrially by heating silica with carbon:
SiO2(s) + 2C(s) → Si(s) + 2CO(g)
Problem: How many grams of silicon can be produced from 500.0 g of SiO₂ with excess carbon? Molar masses: M(Si) = 28.09 g mol-1, M(O) = 16.00 g mol-1, so M(SiO2) = 28.09 + 2(16.00) = 60.09 g mol-1.
Plan: n = mM, apply the 1:1 mole ratio, then m = n × M.
Substitution (dimensional analysis):
n(SiO2) = 500.0 g × 1 mol60.09 g = 8.321 mol
n(Si) = 8.321 mol SiO2 × 1 mol Si1 mol SiO2 = 8.321 mol
m(Si) = 8.321 mol × 28.09 g1 mol = 233.7 g
Answer: about 234 g of silicon — roughly half the starting mass, because oxygen is the other half of SiO₂.
Example 2: What color of light can a silicon solar cell absorb?
Silicon's band gap is about 1.1 eV — the minimum photon energy needed to lift an electron into the conduction band. Photon energy and wavelength are related by:
E = hcλ ⇒ λ= hcE
with h = 6.626 × 10-34 J s, c = 3.00 × 108 m s-1, and 1 eV = 1.602 × 10-19 J.
Problem: What is the longest wavelength (in nm) that can excite an electron across silicon's band gap?
Substitution:
E = 1.1 eV × 1.602 × 10-19 J1 eV = 1.76 × 10-19 J
λ= (6.626 × 10-34 J s)(3.00 × 108 m s-1)1.76 × 10-19 J = 1.13 × 10-6 m
λ= 1.13 × 10-6 m × 109 nm1 m = 1130 nm
Answer: about 1130 nm — near-infrared. Visible light (400–700 nm) carries more energy than the gap, so silicon absorbs it and appears dark gray; longer-wavelength infrared passes through.
Key takeaways
- Metalloids: B, Si, Ge, As, Sb, Te — the staircase elements (Po, At sometimes included).
- Structure: Si and Ge are diamond cubic covalent networks; boron forms B₁₂ icosahedra; all are hard, brittle, high-melting solids.
- Semiconductor = conductivity increases with temperature (opposite of metals); Si band gap ≈ 1.1 eV, Ge ≈ 0.7 eV.
- Doping: group-13 impurities → p-type (holes); group-15 impurities → n-type (extra electrons); p–n junction = diode.
- Metalloid oxides are amphoteric or weakly acidic: SiO₂ reacts with HF and molten base, not other acids; As₂O₃ and Sb₂O₃ react with both acids and bases.
- Compounds are covalent; volatile hydrides (SiH₄, AsH₃, SbH₃) are toxic/pyrophoric and handled only under strict controls.
- Metallic character increases down each group; metalloids are the transition zone of both periodic trends.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
Name the six elements usually classified as metalloids.
Show answer
Boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony (Sb), tellurium (Te); polonium and astatine are sometimes included.
Why does silicon's conductivity increase when heated, while copper's decreases?
Show answer
In a metal, heat makes lattice atoms vibrate more and scatter conduction electrons — conductivity falls. In a semiconductor, heat gives more valence electrons enough energy to jump the band gap — carrier count rises, so conductivity rises.
What structural feature explains boron's hardness, and what is its building block?
Show answer
Boron's atoms form B₁₂ icosahedral clusters bonded into a rigid covalent framework; breaking it requires breaking many strong covalent bonds.
A wafer is doped with a trace of gallium. Is it p-type or n-type, and why?
Show answer
p-type. Gallium (group 13) has three valence electrons, so replacing a four-bonded silicon atom leaves an electron "hole" that acts as a positive charge carrier.
Using SiO2 + 2C → Si + 2CO, how many grams of CO form alongside 50.0 g of Si? (M(CO) = 28.01 g mol-1.)
Show answer
n(Si) = 50.0 / 28.09 = 1.780 mol; the equation gives 2 mol CO per mol Si, so n(CO) = 3.560 mol; m = 3.560 × 28.01 = 99.7 g CO.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Metalloid
- An element with properties between metals and nonmetals
- Band gap
- Energy gap between the filled valence band and empty conduction band
- Semiconductor
- Material whose conductivity rises when energy is added
- Doping
- Deliberately adding tiny amounts of impurity atoms
- Diamond cubic
- Structure where each atom bonds covalently to four neighbors
- Icosahedron
- A 20-faced cluster; B₁₂ icosahedra are boron's building blocks
- Amphoteric
- Able to react as both an acid and a base
- Allotrope
- A different structural form of the same element
Sources & references
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