Chemical Reactivity of Yttrium
Yttrium (Y), a rare earth element, exhibits moderate to high chemical reactivity. It is typically found in compounds in the +3 oxidation state due to the loss of its three valence electrons.
Reactivity with Air
Yttrium metal tarnishes readily upon exposure to air, forming a protective layer of yttrium(III) oxide ($Y_2O_3$). This oxide layer prevents further rapid oxidation at room temperature. When heated in air, yttrium ignites and burns to form yttrium(III) oxide with a bright flame. Powdered yttrium is pyrophoric, meaning it can ignite spontaneously in air at room temperature.
Reactivity with Water
Yttrium reacts with water. It reacts slowly with cold water but reacts more vigorously with hot water or steam to produce yttrium(III) hydroxide ($Y(OH)_3$) and hydrogen gas ($H_2$).
$2Y (s) + 6H_2O (l) \rightarrow 2Y(OH)_3 (s) + 3H_2 (g)$
Reactivity with Acids and Halogens
Yttrium metal readily reacts with most dilute acids to form yttrium(III) salts and hydrogen gas. It also reacts directly with halogens (fluorine, chlorine, bromine, iodine) to form yttrium(III) halides.
Safety Profile of Yttrium
Toxicity
Metallic yttrium is considered to have low acute toxicity. However, yttrium compounds, particularly soluble salts, can be more toxic. Inhalation of yttrium dust or fumes can cause respiratory irritation. Prolonged exposure can lead to lung damage, a condition sometimes referred to as “yttrium lung.” As with many fine metal powders, yttrium powder should be handled with appropriate ventilation and personal protective equipment.
Radioactivity
Naturally occurring yttrium consists entirely of the stable, non-radioactive isotope Yttrium-89. Therefore, yttrium found in nature and in most commercial applications is not radioactive. However, several synthetic radioactive isotopes of yttrium exist, such as Yttrium-90, which is utilized in medical therapies for certain types of cancer.
Flammability
Solid yttrium metal is combustible and can burn at high temperatures. In finely divided powder form, yttrium is pyrophoric, meaning it can spontaneously ignite in air without an external ignition source. This property necessitates careful handling and storage of yttrium powders to prevent fire hazards.
Noteworthy Chemical Reaction
Formation of Yttrium Oxide for Phosphors
One significant chemical reaction involving yttrium is its oxidation to form yttrium(III) oxide ($Y_2O_3$). This reaction is fundamental because yttrium oxide is a key material in many technological applications. For example, yttrium oxide doped with europium ($Y_2O_3:Eu^{3+}$) was historically a crucial component in producing the vibrant red colour in cathode ray tube (CRT) television screens, which were once prevalent in Indian households. The basic reaction is:
$4Y (s) + 3O_2 (g) \rightarrow 2Y_2O_3 (s)$
This yttrium oxide serves as a host matrix for luminescent dopants, allowing for the emission of specific colours of light when excited.