Fermium (Fm) is a synthetic element that does not exist in nature and has no stable forms. Unlike common elements, the number of neutrons in Fermium is not fixed. Since the number of protons defines an element, every Fermium atom always has 100 protons. However, the neutron count varies depending on the specific manufactured variant, or isotope, being discussed.
The Constant: Fermium’s Atomic Number
The defining characteristic of Fermium is its atomic number (Z), which is 100. This specifies that every atomic nucleus of Fermium contains exactly 100 protons. This constant proton count is the chemical fingerprint that distinguishes Fermium from all other elements.
The total mass of the atom’s nucleus is called the Mass Number (A), and it is the sum of the protons and neutrons. To find the number of neutrons in any Fermium isotope, a simple calculation is used: the number of neutrons equals the Mass Number (A) minus the Atomic Number (Z). Therefore, for Fermium, the formula is always Neutrons = Mass Number (A) – 100.
Calculating Neutrons: The Role of Isotopes
The variation in the neutron count is due to isotopes, which are atoms of the same element with differing numbers of neutrons. Scientists have produced Fermium isotopes with Mass Numbers spanning from 242 to 260. Subtracting 100 protons from these values reveals that Fermium can possess anywhere from 142 to 160 neutrons.
The most important isotope for scientific study is Fermium-257, which has the longest half-life of all known variants. Applying the formula (257 minus 100) results in 157 neutrons. This count is often referenced in nuclear chemistry because it corresponds to the most stable form of the element.
Other prominent isotopes have different neutron counts. Fermium-253, which has a half-life of about three days, contains 153 neutrons (253 minus 100). The first isotope discovered during the 1952 thermonuclear test, Fermium-255, has a nucleus composed of 155 neutrons.
Why Fermium is Hard to Study
Fermium is a transuranic element, meaning it lies beyond uranium and is highly radioactive. It was first identified in 1952 after the “Ivy Mike” thermonuclear test, confirming its origin in intense, man-made nuclear events.
The instability of Fermium is defined by the short half-lives of most isotopes, making research difficult. Isotopes heavier than Fermium-257, such as Fermium-258, are prone to immediate spontaneous fission, meaning they instantly break apart. This creates a “fermium gap,” preventing the formation of heavier, longer-lived elements through standard neutron bombardment.
The longest-lived variant, Fermium-257, has a half-life of only about 100 days. Consequently, Fermium can only be produced in minuscule, trace amounts at specialized facilities like high-flux nuclear reactors. The variant with 157 neutrons is the most persistent for observation due to this relatively longer half-life.