metalloid
metalloid, chemical element with properties that fall between those of metals and nonmetals. The term typically refers to a group of between six and eight elements—boron, silicon, germanium, arsenic, antimony, tellurium, and possibly, polonium and astatine—found near the center of the P-block or main block of the periodic table. These elements are classified as metalloids because they share certain physical characteristics with metals, such as luster or moderate conductivity, while they exhibit chemical behavior similar to nonmetals—often forming covalent bonds and acidic oxides.
Properties
Physical properties
- Appearance: They exhibit a metallic luster, giving them a shiny appearance. However, like nonmetals, they are brittle and can shatter under stress.
- Electrical conductivity: They generally act as semiconductors, meaning that they can conduct electricity under certain conditions—this property makes them essential in electronic and photovoltaic devices.
- Thermal conductivity: Their ability to conduct heat falls between that of metals and nonmetals; they are better thermal conductors than nonmetals but not as efficient as metals.
- Density and melting and boiling points: Metalloids usually have densities and melting and boiling points that are intermediate between those of metals and nonmetals, contributing to their classification as a distinct group.
- Allotropy: Metalloids exist in allotropes (multiple structural forms), such as silicon, which appears as amorphous silicon, a brown powder, and crystalline silicon, which has a metallic luster and gray color.
Chemical properties
- Oxidation states: Each metalloid can exhibit more than one oxidation state, which allows these elements to form a wide range of chemical compounds.
- Electronegativity: Metalloids have electronegativity values that fall between those of metals and nonmetals. Metals are generally less electronegative and tend to form ionic compounds, whereas nonmetals are more electronegative and tend to form covalent bonds. Because metalloids sit in the intermediate range, they can form either ionic or covalent bonds, depending on the element with which they react.
- Acid-base behavior: Metals generally react with acids and nonmetals with bases. Metalloids are often amphoteric, meaning that they can react with both acids and bases.
Uses
Metalloids are used in several industries. In electronics silicon and germanium are used as semiconductors in devices such as computer chips, solar cells, and transistors. In glass and ceramics, boron and silicon improve strength and resistance to thermal shock. Borosilicate glass, which includes boron, is used in laboratory equipment and cookware.
Metalloids contribute to alloy production. Silicon is added to aluminum for better casting properties, while boron strengthens steel. Antimony and arsenic are used in lead alloys for batteries, bullets, and solders. Antimony compounds also serve as flame retardants.
In renewable energy silicon and germanium enable photovoltaic cells to convert sunlight into electricity. In agriculture small amounts of boron and silicon promote plant health, though some metalloids, such as arsenic, pose toxicity risks and are regulated accordingly.
Boron is essential for plant cell wall formation but toxic in excess. Other applications include boron fibers in aerospace materials and compounds of tellurium and selenium in thermoelectric devices and rubber processing.