The element possessing exactly 21 protons in its atomic nucleus is Scandium (Sc). The number of protons, known as the atomic number, is the fundamental property that dictates an element’s identity and its placement on the Periodic Table. Every atom with an atomic number of 21 is, by definition, an atom of Scandium.
Defining Scandium: The Result of 21 Protons
Scandium is represented by the chemical symbol Sc and has an average atomic weight of approximately 44.95 atomic mass units. Residing in Group 3 and Period 4, it is the first element classified as a transition metal.
The element’s identity is defined by its electron configuration, which involves all 21 electrons surrounding the nucleus. The ground state configuration is \[Ar] 3d¹ 4s², where the two outermost shells hold three valence electrons. Scandium-45 is the only naturally occurring stable isotope. Its nucleus contains 21 protons and 24 neutrons, resulting in a mass number of 45.
Distinct Physical and Chemical Characteristics
Scandium is a relatively soft, silvery-white metal when freshly prepared. Upon exposure to the atmosphere, it quickly develops a slight yellowish or pinkish tint due to the formation of an oxide layer. It is a lightweight metal, with a density of about 2.99 grams per cubic centimeter, comparable to that of aluminum.
The metal exhibits a high melting point of around 1,541°C, significantly higher than that of light metals like aluminum. Chemically, Scandium is quite reactive, especially when heated, and burns easily in air. It reacts slowly with cold water, but the reaction accelerates when exposed to acids.
The chemical behavior of Scandium is dominated by its singular, stable oxidation state of +3. This state is achieved when the Scandium atom loses all three of its outermost electrons (two from the 4s orbital and one from the 3d orbital). This process results in the formation of the Sc³⁺ ion, which drives almost all of its chemical interactions. While many transition metals exhibit multiple oxidation states, Scandium’s chemistry is less varied due to this preference for the +3 state.
Practical Uses and Natural Occurrence
Scandium is not abundant, occurring sparsely in the Earth’s crust at an estimated concentration of about 22 parts per million. Although sometimes grouped with rare earth elements due to its scarcity, it is technically a transition metal. The element is found in trace amounts in over 800 minerals, but it is primarily concentrated in minerals such as thortveitite, euxenite, and gadolinite.
The most significant commercial use for Scandium is as an additive to create high-performance aluminum-scandium alloys. Even a small amount of Scandium, typically between 0.1% and 0.5%, dramatically increases the strength and lightness of aluminum. These lightweight, durable alloys are valued in the aerospace industry for aircraft components and in high-end sports equipment like bicycle frames and baseball bats.
Scandium is also used in specialized lighting applications, particularly in high-intensity discharge lamps. Scandium iodide is added to these metal halide lamps, which are capable of producing a bright, high-quality light that closely mimics natural sunlight. This capability makes them useful for stadium lighting and for high-definition filming and television production, where accurate color reproduction is important.