Strontium (Sr) is an alkaline earth metal with an atomic number of 38, meaning every atom contains 38 protons. The number of neutrons Strontium has is not fixed. Like many elements, Strontium exists in different forms, called isotopes, which vary only in their neutron count. The neutron count depends entirely on the specific isotope being discussed.
Atomic Structure: Defining Protons, Neutrons, and Atomic Mass
The structure of any atom is defined by three subatomic particles: protons, neutrons, and electrons. Protons carry a positive charge, determine the element’s identity, and reside in the nucleus alongside the neutral neutrons. The number of protons is the atomic number (Z), which is fixed at 38 for all Strontium atoms.
The total number of particles in the nucleus—protons plus neutrons—is known as the mass number (A). The mass number identifies a specific isotope, such as Strontium-88. To find the number of neutrons in a given isotope, one subtracts the atomic number (Z) from the mass number (A). This relationship is expressed as: Neutrons = A – Z. For Strontium, this calculation is Neutrons = A – 38.
The Role of Isotopes in Determining Neutron Count
An isotope is a variation of a chemical element that has the same number of protons but a different number of neutrons. Since the number of protons is constant, all Strontium isotopes share the same chemical properties. The varying neutron count, however, results in a different atomic mass for each isotope. This variability is why a single neutron number cannot be given.
The stability of an atom is directly influenced by the ratio of neutrons to protons in its nucleus. Isotopes with an unstable ratio are radioactive and decay over time, while those with a stable ratio are the forms found in nature.
Calculating Neutrons for Strontium’s Stable Forms
Strontium has four naturally occurring stable isotopes: Strontium-84, Strontium-86, Strontium-87, and Strontium-88. These four isotopes constitute virtually all the Strontium found in the Earth’s crust. The neutron count for each is calculated by subtracting the atomic number (38) from the isotope’s mass number.
Strontium-84, the least common stable form, has a mass number of 84, resulting in 46 neutrons (84 – 38 = 46). Strontium-86, which makes up about 9.86% of natural Strontium, contains 48 neutrons (86 – 38 = 48).
Strontium-87 has 49 neutrons (87 – 38 = 49). This isotope is notable because its abundance can vary slightly due to its formation from the decay of radioactive Rubidium-87.
Strontium-88 is the most common form, making up approximately 82.58% of all natural Strontium. This isotope contains 50 neutrons (88 – 38 = 50), which is the neutron count most representative of the element as a whole. The different neutron numbers lead to an average atomic weight for Strontium of about 87.62, reflecting the weighted average of these stable isotopes.
Beyond Stability: Radioactive Strontium Isotopes
Strontium also has numerous unstable, radioactive isotopes that are not naturally abundant. Strontium-90 is the most significant among these due to its presence in nuclear fallout and its long half-life of nearly 29 years.
Strontium-90 has a mass number of 90, meaning it possesses 52 neutrons (90 – 38 = 52). This isotope is produced primarily as a byproduct of nuclear fission in reactors and weapons testing. Because Strontium behaves chemically like Calcium, Strontium-90 can be absorbed by the body and accumulate in bones and teeth.