Atomic Properties of La
- Atomic Number of La
- 57
- Atomic Mass of La
- 138.9100 u
- Electron Configuration
- [Xe] 5d1 6s2
- Electronegativity
- 1.10
- Block
- f-block
- Group
- —
- Period
- 6
In 1839, Carl Gustaf Mosander separated a new oxide from material then regarded as cerium oxide. He named the concealed element lanthanum from the Greek lanthanein, “to lie hidden.” The discovery set an enduring theme for rare-earth chemistry: substances that looked alike could contain distinct elements, separated only through patient chemical work.
Lanthanum now stands at the entry point to the whole eccentric family commonly called the lanthanides. Its atoms usually form La3+ ions, and its compounds are typically colorless because the ion has no 4f electrons. That quiet electronic detail supports some conspicuously useful materials.
Lanthanum-containing optical glass can combine a high refractive index with controlled dispersion, helping designers build compact, sharp camera lenses. Nickel-metal hydride batteries use lanthanum-rich alloys to store hydrogen at one electrode. Lanthanum compounds also serve in petroleum-cracking catalysts, while lanthanum oxide adjusts glass and ceramic properties. These uses belong to engineered compounds and alloys, not to an imaginary all-purpose pure metal.
Natural lanthanum is almost entirely stable lanthanum-139, with a tiny share of extremely long-lived radioactive lanthanum-138. The reactive silvery metal tarnishes in air and is handled with appropriate controls.
Lanthanum makes a fitting host for the Luminaries. It was hidden inside a supposed single substance, then helped reveal that an entire chemical neighborhood was waiting behind the door. Today it still works through mixtures, lenses, electrodes, and catalysts where its name is less visible than its effect.