Lanthanum, symbolized as La, is a soft, silvery-white metal belonging to the lanthanide group, often classified as a rare earth element. It is chemically reactive and challenging to isolate from its ores, despite not being particularly rare in the Earth’s crust. Lanthanum has an atomic number of 57, meaning every atom contains 57 protons in its nucleus.
The Measured Atomic Mass of Lanthanum
The officially recognized standard atomic weight for Lanthanum is 138.90547 unified atomic mass units (u). This value represents the average mass of a Lanthanum atom as it is found in nature. The unified atomic mass unit is a standard measure used in chemistry and physics, defined as one-twelfth of the mass of a single carbon-12 atom.
How Atomic Mass is Determined
The atomic mass value is calculated as a weighted average that accounts for the different forms of the element found in nature. Atoms of the same element that have an identical number of protons but vary in the number of neutrons are called isotopes. Lanthanum naturally occurs as a mixture of two principal isotopes, Lanthanum-139 (\(^{139}\text{La}\)) and Lanthanum-138 (\(^{138}\text{La}\)).
Lanthanum-139 makes up the vast majority of natural Lanthanum, approximately 99.911%. The remaining fraction, about 0.089%, is the minor isotope Lanthanum-138, which is a primordial radioactive isotope.
The standard atomic weight is derived by multiplying the exact mass of each isotope by its relative abundance and then summing the results. Since Lanthanum-139 is overwhelmingly abundant, the final weighted average mass is extremely close to 139. The presence of the lighter Lanthanum-138 slightly pulls the final average mass down to 138.90547 u.
Lanthanum’s Role in Modern Technology
Lanthanum is used extensively in several high-technology and consumer applications.
Energy Storage
One recognized use is in energy storage, specifically in nickel-metal hydride (NiMH) batteries. Lanthanum-nickel alloys are a component of the anode in these batteries, commonly found in hybrid electric vehicles. The element’s ability to efficiently absorb and release hydrogen allows these batteries to store a high charge capacity.
Optical Glass
Lanthanum oxide is valued for its unique optical properties, making it valuable in the production of high-quality glass. This compound is added to glass mixtures to increase the refractive index while lowering the dispersion. The resulting product is used to create advanced lenses for cameras, telescopes, and microscopes, allowing for sharper images.
Industrial Catalysts
Lanthanum compounds serve an important function as specialized catalysts in industrial processes. Lanthanum oxide is a key additive in fluid catalytic cracking (FCC) units within petroleum refineries. This addition increases the efficiency of the cracking process, which breaks down heavy crude oil fractions into valuable products like gasoline and diesel fuel.