Elements are the fundamental building blocks of matter, each with unique characteristics. They are systematically organized on the periodic table based on their atomic structure and properties.
The Atomic Mass of Lanthanum
Lanthanum, symbolized as La on the periodic table, has an atomic mass of approximately 138.91 atomic mass units (amu). Atomic mass represents the average mass of an atom of a particular element, taking into account the varying masses of its naturally occurring forms.
The reason this number is not a whole number is because most elements, including lanthanum, exist as a mixture of isotopes. Isotopes are atoms of the same element that have the same number of protons but differ in their number of neutrons. Therefore, the atomic mass listed on the periodic table reflects a weighted average of these different isotopic masses, based on their relative abundance in nature. This average provides a practical value for chemical calculations involving the element.
How Atomic Mass is Determined
The concept of isotopes is central to understanding how atomic mass values are established. For instance, naturally occurring lanthanum is primarily composed of two isotopes: lanthanum-139, which is stable and accounts for about 99.9% of all natural lanthanum, and lanthanum-138, a radioactive isotope present in much smaller quantities, approximately 0.089%. Each of these isotopes has a specific mass due to its unique neutron count.
Scientists determine the overall atomic mass by calculating a weighted average. This involves multiplying the exact mass of each isotope by its natural abundance and then summing these results. These precise isotopic masses and their natural abundances are empirically measured through an analytical technique called mass spectrometry. Mass spectrometry separates atoms based on their mass-to-charge ratio, allowing researchers to identify the different isotopes present in a sample and quantify their relative proportions.
Lanthanum: A Unique Element
Beyond its atomic mass, lanthanum itself is an intriguing element with distinct characteristics. It is classified as a rare earth element and is the first in the series known as the lanthanides, serving as a prototype for this group of chemically similar elements. In its pure form, lanthanum is a soft, silvery-white metal that is ductile and malleable. It is also highly reactive, readily tarnishing when exposed to air and reacting with water.
Lanthanum’s properties make it valuable in various modern applications. It is used in the production of high-refraction optical glasses for camera lenses, enhancing clarity and performance. Its compounds are also components in nickel-metal hydride batteries found in hybrid electric vehicles and in lighter flints as part of an alloy called Mischmetal. Lanthanum serves as a catalyst in petroleum refining processes and contributes to the bright, white light of carbon arc lamps used in studio lighting and projectors.