Terbium, symbolized as Tb, is a silvery-white, rare earth element that belongs to the lanthanide series on the periodic table. It is never found in nature in its pure form but is instead extracted from various minerals like monazite and euxenite. The question of how many neutrons Terbium has does not have a single fixed answer because the number of neutrons is not constant for any element. This count depends entirely on the specific form, or isotope, of the Terbium atom being considered.
The Stable Isotope Answer: Terbium-159
When people ask about the neutron count of an element, they are typically referring to the most common, naturally occurring, and stable version. For Terbium, this stable form is Terbium-159 (Tb-159), which accounts for 100% of all Terbium found in nature, making it a monoisotopic element.
To determine the number of neutrons in Tb-159, one must first know the number of protons. Terbium’s atomic number (Z) is 65, meaning every Terbium atom contains exactly 65 protons in its nucleus. The number 159 is the mass number (A), representing the total count of protons and neutrons combined.
By subtracting the number of protons from the mass number, the number of neutrons is determined. For the stable Tb-159 isotope, the calculation is \(159 – 65 = 94\). Therefore, Terbium-159 has 94 neutrons. This combination of 65 protons and 94 neutrons results in a balanced nucleus that does not undergo radioactive decay.
Calculating Neutrons: Atomic Mass and Atomic Number
Determining the neutron count for any atom uses the fundamental equation: Neutrons (N) = Mass Number (A) – Atomic Number (Z). This calculation is based on the composition of the atom’s nucleus, which is a dense core made up of protons and neutrons, collectively known as nucleons.
The Atomic Number (Z) is the count of protons, which dictates the element’s identity; any atom with 65 protons is Terbium. This number is fixed for every atom of Terbium. Protons carry a positive electrical charge and define the element’s chemical behavior.
The Mass Number (A) represents the total count of these nucleons (protons and neutrons). Since the mass of an electron is negligible, the Mass Number dictates the atomic mass. This number can change between different atoms of the same element, leading to the existence of isotopes. Combining the Mass Number (A) of a specific isotope with the constant Atomic Number (Z) reveals the precise number of neutrons (N).
Isotopic Variation and Neutron Counts in Terbium
While Terbium-159 is the only stable form, scientists have characterized a large range of other isotopes. These isotopes are created artificially and are often unstable, meaning they are radioactive and decay over time. Terbium’s known isotopes range widely, from Tb-135 up to Tb-174, demonstrating a significant variation in neutron count.
This difference in neutron count defines one isotope from another, though all retain 65 protons. For example, the artificially produced isotope Terbium-158 (Tb-158) has a mass number of 158, resulting in 93 neutrons (\(158 – 65\)). Terbium-161 (Tb-161) has 96 neutrons (\(161 – 65\)), and is currently being researched for potential cancer therapy.
The varying number of neutrons determines the stability of the isotope and its half-life. Isotopes with fewer than 94 neutrons, such as Tb-158, typically decay through electron capture, resulting in isotopes of Gadolinium. Isotopes with more than 94 neutrons, such as Tb-161, generally undergo beta-minus decay, transforming into isotopes of Dysprosium. This ability to create specific neutron counts allows scientists to tailor Terbium isotopes for specialized applications, such as medical imaging and targeted radiation therapy.