Cerium (Ce) is a soft, silvery metal belonging to the lanthanide series, often called rare earth elements. Cerium is the most abundant of these metals, making it relatively common in the Earth’s crust. Every cerium atom is defined by its atomic number, 58, meaning it contains 58 protons within its nucleus.
Atomic Building Blocks
The number of neutrons in any atom depends on two fundamental measurements: the Atomic Number (\(Z\)) and the Mass Number (\(A\)). The Atomic Number (\(Z\)) is the count of protons found in the nucleus, which dictates the element’s chemical identity. The Mass Number (\(A\)) represents the total number of particles contained within the atom’s nucleus, including both protons and neutrons. Since the mass of electrons is negligible, the Mass Number provides an integer approximation of an atom’s mass. This figure is always a whole number and is specific to a single isotope of an element.
Determining the Neutron Count
To find the number of neutrons (\(N\)) in an atom, the Mass Number (\(A\)) is subtracted from the Atomic Number (\(Z\)): \(N = A – Z\). Cerium’s Atomic Number (\(Z\)) is consistently 58. The standard atomic weight of cerium is approximately \(140.116\) atomic mass units, which is the weighted average of all its naturally occurring forms. To use the formula, this average weight is rounded to the nearest whole number to find the most common Mass Number (\(A\)), which is \(140\). Subtracting the atomic number from this mass number (\(140 – 58\)) yields \(82\). Therefore, the most common form of cerium, cerium-140, contains 82 neutrons.
Cerium’s Range of Isotopes
Although 82 is the most frequent count, the number of neutrons in a cerium atom is not fixed, as the element exists in various forms called isotopes. Isotopes are atoms of the same element that share the same number of protons (\(Z\)) but possess a different number of neutrons, resulting in a different Mass Number (\(A\)). Naturally occurring cerium is composed of four stable isotopes:
- Cerium-140 (\(\text{Ce}-140\)), the most abundant (88.45%), contains 82 neutrons.
- Cerium-142 (\(\text{Ce}-142\)), the second most common (11.11%), has 84 neutrons.
- Cerium-138 (\(\text{Ce}-138\)) has 80 neutrons.
- Cerium-136 (\(\text{Ce}-136\)) contains 78 neutrons.
This variation in neutron number confirms that the concept of a single, fixed neutron count for an element is generally inaccurate.
Practical Uses of Cerium
Cerium is valued for its unique chemical properties, which lead to a wide range of industrial applications. Cerium oxide, also known as ceria, is the most common compound used and serves as an important polishing agent in the optics industry. Its fine particles are the preferred material for polishing high-quality glass surfaces, including mirrors and precision lenses.
Cerium is also a prominent component in catalysis, particularly in the automotive industry’s catalytic converters. Its ability to easily switch between two different oxidation states, \(\text{Ce}^{3+}\) and \(\text{Ce}^{4+}\), allows it to store and release oxygen, necessary for reducing harmful vehicle emissions. This process helps convert carbon monoxide and nitrogen oxides into less toxic substances. Additionally, cerium is alloyed with other metals to improve material performance, such as in Mischmetal, a mixture used in lighter flints for its pyrophoric nature.