When scientists refer to a specific form of an element, they use a naming system that communicates the atom’s inner structure. Nomenclature like “Polonium-218” is a concise way to describe the composition of the atomic nucleus. Understanding this system is fundamental to grasping how different types of atoms relate. The number following the element name identifies how heavy that particular atom is compared to its counterparts.
The Direct Answer: What the Number 218 Represents
The number 218 in Polonium-218 is the mass number, representing the total count of protons and neutrons (nucleons) found within the atomic nucleus. This value is significant because the mass of an atom is almost entirely concentrated in its nucleus, giving the mass number as a good approximation of the atom’s relative atomic weight.
For Polonium-218, the number 218 signifies that the nucleus contains 218 combined protons and neutrons. This count distinguishes Polonium-218 from other versions of the element and dictates its stability and how it undergoes radioactive transformation.
The Constant Identity: Defining Polonium
While the mass number (218) is variable, Polonium has a constant, defining characteristic: its atomic number. The atomic number is the precise count of protons in the nucleus, and for Polonium, this number is always 84. Every atom with 84 protons is, by definition, Polonium, which is why the element is sometimes symbolized as \({}_{84}\text{Po}\).
The number of protons determines the element’s chemical identity and how it interacts with other substances. Polonium’s 84 protons dictate its position on the periodic table and its chemical properties, such as being a radioactive metal. This unchanging number of protons contrasts with the variable mass number, but it ensures that Polonium-218 is chemically identical to Polonium-210, despite their different masses.
Understanding Isotope Variation
The difference between Polonium’s fixed identity (84 protons) and the variable mass number (218) introduces the concept of an isotope. Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons.
For Polonium-218, subtracting the atomic number (84 protons) from the mass number (218 total nucleons) reveals that this specific atom contains 134 neutrons. Other Polonium isotopes, such as Polonium-210, also have 84 protons but only 126 neutrons, resulting in a mass number of 210. This variation in neutron count among isotopes is what causes their differing mass numbers and, subsequently, their distinct nuclear stabilities. Polonium-218 is just one of 42 known variations of Polonium, all of which are radioactive because Polonium has no stable isotopes.
The Role of Polonium-218 in Nature
Polonium-218 holds a specific and short-lived position within the natural radioactive decay series of Uranium-238. This isotope is produced as an intermediate product when Radon-222, a radioactive gas found naturally in the environment, undergoes alpha decay.
The significance of Polonium-218 is defined by its extremely short half-life, which is approximately 3.1 minutes. This means that half of any given sample of Polonium-218 will decay into Lead-214 within this brief period. This rapid decay makes Polonium-218 a highly energetic and potent alpha-particle emitter.
Because it quickly attaches to airborne particles, Polonium-218 is often a primary concern in environmental health studies related to indoor Radon exposure. It is one of the short-lived decay products that can be inhaled, where its rapid alpha decay within the lungs poses a localized radiation hazard.