Metalloid
Metalloids are chemical elements that exhibit a mixture of metallic and non-metallic characteristics, resulting in properties that lie between those of metals and non-metals. The term is derived from the Latin metallum meaning metal, and the Greek oeides meaning resembling in form. Although widely used in chemical literature, the concept lacks a universally accepted definition, and the classification of elements as metalloids varies among scientific authors. Despite this ambiguity, several elements consistently appear in metalloid lists, and the concept remains important in understanding periodic trends, material properties and technological applications.
Typical metalloids include boron, silicon, germanium, arsenic, antimony and tellurium. Less frequently, carbon, aluminium, selenium, polonium and astatine are added. These elements occupy a diagonal region of the p-block of the periodic table, forming a notional dividing zone between metals and non-metals. Their intermediate properties make them essential in fields such as semiconductor technology, materials science and catalysis.
General Characteristics
Metalloids typically display a metallic sheen yet are mechanically brittle. Their electrical conductivity is moderate, falling between that of insulating non-metals and highly conducting metals. Many metalloids behave as semiconductors, with conductivity that can be modified by temperature, impurities or structural form. Chemically, they can form alloys with metals, while also forming covalent compounds with non-metals. Their oxides are often amphoteric or weakly acidic, reflecting intermediate acid–base behaviour.
Physical and chemical properties commonly associated with metalloids include:
- Metallic appearance combined with poor mechanical malleability.
- Moderate electrical conductivity, enhanced by doping.
- Intermediate electronegativity and ionisation energies.
- Capacity to form both metallic alloys and covalent compounds.
These hybrid characteristics underpin their importance in electronics, optical materials, pyrotechnics and biological applications.
Classification and Challenges
The classification of metalloids is not universally fixed, largely because metallic and non-metallic properties do not form sharply defined categories. Many elements show mixed behaviour, and only those near the margins—with no dominant set of metallic or non-metallic traits—are typically classed as metalloids.
The most commonly accepted metalloids are:
Occasionally the following are included:
Authors differ in their choices due to the qualitative nature of the criteria used. Some classifications are judgement-based, relying on impressions of mixed properties, while others use numerical thresholds such as electronegativity, ionisation energy, atomic packing factor or the Goldhammer–Herzfeld ratio. Even within these frameworks, anomalies arise. For example, boron is sometimes excluded for its strongly non-metallic chemistry; tellurium is occasionally omitted due to the dominance of its metallic behaviour; and the status of polonium and astatine remains uncertain owing to limited empirical data.
Different researchers have proposed varying lists: some recognise as few as four metalloids (germanium, arsenic, antimony and tellurium), while others extend the category to include up to twelve elements. On average, about seven elements appear across most compilations, reflecting broad but imprecise agreement.
Judgement-Based Definitions
Historical and contemporary authors have offered qualitative interpretations of metalloids:
- Elements situated between metals and non-metals in the periodic table that exhibit mixed traits.
- Elements difficult to classify definitively as either metallic or non-metallic.
- Elements that are “in-between” or transitional in their behaviour, showing both cation-forming ability and covalent bonding tendencies.
These definitions emphasise difficulty of categorisation as a core attribute. Many elements display both metallic and non-metallic properties, yet only those without a clear predominance of either set of features tend to be labelled as metalloids.
An illustrative comparison is gold, which also shows mixed behaviour—combining high electrical conductivity with unusually high electronegativity and the ability to form anionic species—yet it is still considered a metal due to the dominance of metallic characteristics. Metalloids, by contrast, lack such a dominant identity.
Criteria-Based Definitions
Quantitative or semi-quantitative criteria have also been used to identify metalloids. These criteria include:
- Electronegativity values typically between approximately 1.8 and 2.2.
- Ionisation energies around 200 kcal mol⁻¹ (≈837 kJ mol⁻¹).
- Atomic packing efficiencies between roughly 34% and 41%.
- Goldhammer–Herzfeld ratios near unity, indicating borderline metallic behaviour.
- Atomic conductance lower than that of metals but higher than that of insulating non-metals.
- Coordination numbers rarely exceeding eight, distinguishing them from true metals.
No single criterion is sufficient to define all metalloids; instead, the classification generally considers a combination of features. Attempts at strict categorisation are often regarded as arbitrary due to exceptions and transitional behaviour.
Historical Development
The term metalloid originally referred to non-metals in early chemical literature. Its modern meaning—denoting elements with intermediate or hybrid properties—became established during the mid-twentieth century, particularly between 1940 and 1960. Over time, the term semimetal was also applied, though this usage is discouraged in chemistry because semimetal has a specific meaning in solid-state physics relating to electronic band structures. In that context, only arsenic and antimony qualify as semimetals, despite several other elements being chemically classified as metalloids.
The evolving definitions reflect the gradual refinement of chemical classification, the development of electronic theories of bonding, and the recognition that many elements do not fit neatly into the binary categories traditionally used in the periodic table.
Properties and Behaviour of Notable Metalloids
Several metalloids possess properties that define key chemical and technological applications:
- Boron is highly hard, forms strong covalent networks and is essential in borosilicate glass and detergent chemistry.
- Silicon is the cornerstone of semiconductor technology and forms the basis of modern electronics.
- Germanium was historically vital in early transistors and continues to be used in fibre-optic systems.
- Arsenic plays roles in alloys, pesticides and semiconductor doping.
- Antimony is widely used in flame retardants, solders and thermoelectric materials.
- Tellurium is important in specialised alloys, solar panels and optical storage media.
Their compounds often exhibit amphoteric behaviour or unusual bonding features that contribute to catalytic activity, electronic functionality and material design.
Applications and Industrial Relevance
Metalloids and their compounds are important in numerous technological fields due to their intermediate or tunable properties. Common uses include:
- Semiconductors and electronics, especially silicon, germanium and arsenic compounds.
- Optoelectronics and optical storage, such as tellurium-based phase-change materials.
- Alloys and metallurgical additives, improving hardness, strength or conductivity.
- Flame retardants, notably antimony trioxide.
- Glass, ceramics and enamels, where boron and silicon compounds provide structural and thermal advantages.
- Biological and medical agents, where arsenic and boron compounds have niche therapeutic and diagnostic roles.
- Pyrotechnics, where the combustion characteristics of metalloids provide colour and stability.
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