Radon is a colorless, odorless, and tasteless noble gas that arises naturally from the radioactive decay of uranium found in soil and rock. This gas is a source of radiation exposure, and understanding its atomic structure is necessary to comprehend its properties. Determining the neutron count requires first establishing the fixed identity of the element before applying a calculation based on mass.
The Constant: Defining Radon by its Protons
The identity of any chemical element is defined by the number of protons contained within its nucleus. This number is known as the atomic number, represented by the symbol \(Z\). Every atom of Radon must possess the same number of protons to be classified as Radon.
Radon’s atomic number is 86, meaning every Radon atom has exactly 86 protons in its nucleus. These 86 positively charged particles are the constant, defining feature of the element. The atomic number dictates the element’s position on the periodic table. This fixed count of 86 protons is the starting point for calculating the neutron count.
Calculating the Neutron Count
The total number of particles within an atom’s nucleus—the protons and neutrons combined—is called the mass number, symbolized by \(A\). Neutrons are subatomic particles with no electrical charge. The fundamental relationship linking these three counts is a simple subtraction problem.
To find the number of neutrons (\(N\)), one must subtract the atomic number (\(Z\)) from the mass number (\(A\)). The formula is expressed as \(N = A – Z\). Since the mass number (\(A\)) is not fixed for an element, the specific version of Radon must be known before the neutron count can be accurately determined. The neutron count will vary directly with the mass number (\(A\)).
The Primary Answer: Neutrons in Radon-222
The most common and most stable form of the element encountered in the natural environment is Radon-222 (\(\text{Rn}^{222}\)). This particular atom is the direct product of the decay chain of uranium-238 and is responsible for the majority of indoor Radon exposure. The number 222 in its name is its mass number (\(A\)).
Applying the calculation to this specific atom yields the most frequent answer to the question of Radon’s neutron count. Using the fixed atomic number of 86 and the mass number of 222, the formula is \(222 – 86 = 136\). Therefore, the most typical Radon atom, Radon-222, contains 136 neutrons in its nucleus. This specific neutron count allows Radon-222 to have a half-life of about 3.8 days.
Understanding Radon’s Different Forms (Isotopes)
The reason the neutron count is not a single, fixed number is that elements exist in different structural forms known as isotopes. Isotopes are atoms of the same element that share the same number of protons but contain a varying number of neutrons. This difference in neutron count results in a different mass number for each isotope.
While Radon-222 is the most stable, there are other naturally occurring isotopes, each with a unique neutron count. For instance, Radon-220 (Thoron) has a mass number of 220, meaning it has 134 neutrons (\(220 – 86\)). Radon-219 (Actinon) has a mass number of 219, resulting in 133 neutrons (\(219 – 86\)). The variation in the number of neutrons drastically affects the stability of the nucleus. The addition or subtraction of just two neutrons reduces the half-life of Radon-220 to approximately 55 seconds, compared to the multiple days of Radon-222.