#periodic table

• Actinide — A metallic and radioactive chemical element belonging to the actinide (or actinoid) series, a stabilized structural group of 15 heavy elements located in the periodic table from actinium (atomic number 89) to lawrencium (atomic number 103). All these elements belong to the 'f' block (5f series) of transition metals, characterized by high nuclear instability, high density, and electrochemical properties closely correlated with those of actinium.
• Actinides — Plural form for the neuter noun 'actinid'; a homogeneous structural group in the periodic table consisting of 15 metallic and radioactive chemical elements, with atomic numbers ranging from 89 (actinium) to 103 (lawrencium). Located in the 'f' block of transition elements in the 7th period, they exhibit similar chemical properties due to the progressive filling of the 5f electron subshell, being recognized for their high density, substantial nuclear instability, and capacity to undergo fission.
• Actinium — A radioactive, silvery-white metallic chemical element with atomic number 89 and symbol Ac, which initiates the actinide series in the periodic table. Discovered in 1899 by André-Louis Debierne, actinium is extremely rare in the Earth's crust, occurring in trace amounts within uranium ores. Structurally, it crystallizes in a face-centered cubic system, is highly reactive, and emits an intense bluish glow in the dark due to the ionization of the surrounding air.
• Actinoid — The official systematic name recommended by IUPAC for any chemical element within the actinide series, encompassing the 15 metallic and radioactive elements with atomic numbers from 89 (actinium) to 103 (lawrencium). As an adjective, it describes a chemical behavior or atomic structure resembling that of actinium, characterized by the progressive filling of the 5f electron subshell, very high density, and electrochemical properties specific to heavy metals.
• Ag — The official chemical symbol used internationally for the element silver, derived from its Latin name 'Argentum'. It represents a precious transition metal, bright white in color, characterized by the highest electrical and thermal conductivity of all elements, widely used in electronics, jewelry, medicine, and coinage due to its structural stability.
• Alkaline earth — Pertaining to or characteristic of the group of bivalent chemical elements in group 2 of the periodic table (such as beryllium, magnesium, calcium, strontium, barium, and radium). The name originates from the fact that the oxides of these metals exhibit pronounced alkaline properties and were historically called 'earths' due to their insolubility and high resistance to elevated temperatures.
• Aluminum — A chemical element with atomic number 13 and symbol Al; it is a silvery-white, lightweight metal in the boron group, remarkable for its low density, malleability, high electrical and thermal conductivity, and excellent corrosion resistance due to a protective oxide layer.
• Americium — A synthetic, highly radioactive chemical element of the actinide series, with atomic number 95 and symbol Am. This silver-colored structural transuranium metal, originally obtained by bombarding plutonium with neutrons, is utilized in precision industrial applications, such as household smoke detectors and alpha radiation sources for laboratory analyses.
• Antimony — A chemical element with atomic number 51 and symbol Sb, located in group 15 of the periodic table, being a lustrous, silvery-white metalloid that is brittle and possesses low thermal and electrical conductivity. In a structural and industrial sense, it is widely utilized in manufacturing antifriction alloys, battery plates, semiconductor materials, and as a flame retardant agent.
• Argon — A chemical element with atomic number 18 and symbol Ar, representing a noble gas that is colorless, odorless, and tasteless, constituting approximately 0.93% of the Earth's atmosphere. In a structural sense, argon has a fully populated and stable outer electron shell (octet), giving it remarkable chemical inertness; it is widely used in industry as a protective atmosphere for gas tungsten arc welding, electronics manufacturing, and incandescent light bulb filling.
• Arsenic — A chemical element with atomic number 33 and symbol As, located in group 15 of the periodic table, classified as a metalloid with properties intermediate between metals and non-metals. It occurs in several allotropic forms (the most stable being grey arsenic, with a metallic luster), is a semiconductor used in microelectronics and optoelectronics, and, in compound forms, is historically recognized as an extremely dangerous cellular toxin with severe industrial uses.
• Astatine — A highly unstable radioactive chemical element of the halogen group, with atomic number 85 and symbol At; it is the rarest naturally occurring element in the Earth's crust, produced only transiently through the decay of uranium and thorium, and due to its extremely short half-life (a maximum of 8.1 hours for its most stable isotope), its physical properties are studied almost exclusively through artificial synthesis techniques in particle accelerators.
• At — The official chemical symbol for the element astatine, an extremely rare and unstable radioactive halogen with atomic number 85. In modern periodic tables, At represents the heaviest characterized element in the halogen group, whose physical and chemical properties are theoretically deduced or microscopically observed in high-energy nuclear physics laboratories due to its very short half-life.
• Nitrogen — A gaseous chemical element, colorless, odorless, and tasteless, with atomic number 7 and symbol N, constituting approximately 78% of the Earth's atmosphere by volume; it is a fundamental component of amino acids, proteins, and nucleic acids, essential for all living organisms and widely used in industry in liquid form as a cryogenic agent or within chemical fertilizers.