Standard atomic weight in the context of Atomic mass constant

In chemistry, the molar mass (M) (sometimes called molecular weight or formula weight, but see related quantities for usage) of a chemical substance (element or compound) is defined as the ratio between the mass (m) and the amount of substance (n, measured in moles) of any sample of the substance: M = m/n. The molar mass is a bulk, not molecular, property of a substance. The molar mass is a weighted average of many instances of the element or compound, which often vary in mass due to the presence of isotopes. Most commonly, the molar mass is computed from the standard atomic weights and is thus a terrestrial average and a function of the relative abundance of the isotopes of the constituent atoms on Earth.

The molecular mass (for molecular compounds) and formula mass (for non-molecular compounds, such as ionic salts) are commonly used as synonyms of molar mass, as the numerical values are identical (for all practical purposes), differing only in units (dalton vs. g/mol or kg/kmol). However, the most authoritative sources define it differently. The difference is that molecular mass is the mass of one specific particle or molecule (a microscopic quantity), while the molar mass is an average over many particles or molecules (a macroscopic quantity).

Uranium (₉₂U) is a naturally occurring radioactive element (radioelement) with no stable isotopes. It has two primordial isotopes, uranium-238 and uranium-235, that have long half-lives and are found in appreciable quantity in Earth's crust. The decay product uranium-234 is also found. Other isotopes such as uranium-233 have been produced in breeder reactors. In addition to isotopes found in nature or nuclear reactors, many isotopes with far shorter half-lives have been produced, ranging from U to U (except for U). The standard atomic weight of natural uranium is 238.02891(3).

Natural uranium consists of three main isotopes, U (99.2739–99.2752% natural abundance), U (0.7198–0.7202%), and U (0.0050–0.0059%). All three isotopes are radioactive (i.e., they are radioisotopes), and the most abundant and stable is uranium-238, with a half-life of 4.463×10 years (about the age of the Earth).

In chemistry, transferability is the assumption that a chemical property that is associated with an atom or a functional group in a molecule will have a similar (but not identical) value in a variety of different circumstances. Examples of transferable properties include:

• Electronegativity
• Nucleophilicity
• Chemical shifts in NMR spectroscopy
• Characteristic frequencies in Infrared spectroscopy
• Bond length and bond angle
• Bond energy

Transferable properties are distinguished from conserved properties, which are assumed to always have the same value whatever the chemical situation, e.g. standard atomic weight.

Oganesson (₁₁₈Og) is a synthetic element created in particle accelerators, and thus a standard atomic weight cannot be given. Like all synthetic elements, it has no stable isotopes. The first and only isotope to be synthesized was Og in 2002 and 2005; it has a half-life of 0.7 milliseconds.

Bismuth (₈₃Bi) has 41 known isotopes, ranging from Bi to Bi. Bismuth has no stable isotopes, but does have one naturally occurring, very long-lived isotope; thus, the standard atomic weight can be given from that isotope, bismuth-209. Though it is now known to be radioactive, it may still be considered practically stable because it has a half-life of 2.01×10 years, which is more than a billion times the age of the universe.

Besides Bi, the most stable bismuth radioisotopes are Bi with a half-life of 3.04 million years, Bi with a half-life of 368,000 years and Bi, with a half-life of 31.22 years, none of which occur in nature. All other isotopes have half-lives under 15 days, most under two hours. Of naturally occurring radioisotopes, the most stable is radiogenic Bi with a half-life of 5.012 days. Bi is unusual for being a nuclear isomer with a half-life many orders of magnitude longer than that of the ground state.

Plutonium (₉₄Pu) is an artificial element, except for trace quantities resulting from neutron capture by uranium, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no stable isotopes. It was synthesized before being found in nature, with the first isotope synthesized being Pu in 1940. Twenty-two plutonium radioisotopes have been characterized. The most stable are Pu with a half-life of 81.3 million years; Pu with a half-life of 375,000 years; Pu with a half-life of 24,110 years; and Pu with a half-life of 6,561 years. This element also has eight meta states; all have half-lives of less than one second.

The known isotopes of plutonium range from Pu to Pu. The primary decay modes before the most stable isotope, Pu, are spontaneous fission and alpha decay; the primary mode after is beta emission. The primary decay products before Pu are isotopes of uranium and neptunium (not considering fission products), and the primary decay products after are isotopes of americium.

Neptunium (₉₃Np) is usually considered an artificial element, although trace quantities are found in nature, so a standard atomic weight cannot be given. Like all trace or artificial elements, it has no stable isotopes. The first isotope to be synthesized and identified was Np in 1940, produced by bombarding
U with neutrons to produce
U, which then underwent beta decay to
Np.

Trace quantities are found in nature from neutron capture reactions by uranium atoms, a fact not discovered until 1951.