Atomic Weights

notes
llm
physics
Author

Stephen J. Mildenhall

Published

2025-11-06

Modified

2025-11-06

Elements by Number of Stable Isotopes

Based on the number of stable isotopes (nuclides that have not been observed to decay), elements can be categorized as follows.

The 26 elements with only one stable isotope are known as monoisotopic elements. Elements with an odd atomic number are much more likely to be monoisotopic.

The element with the most stable isotopes is Tin (Sn), which has 10. Xenon (Xe) is second with 9 (though some sources list 8 or 9 depending on the definition of “stable” vs. “primordial”).

Elements by number of stable isotopes.
Number Count Examples
1 26 Beryllium (Be), Fluorine (F), Sodium (Na), Aluminium (Al), Phosphorus (P), Cobalt (Co), Gold (Au)
2 16 Hydrogen (H), Lithium (Li), Boron (B), Nitrogen (N), Chlorine (Cl), Potassium (K), Copper (Cu)
3 5 Oxygen (O), Neon (Ne), Magnesium (Mg), Silicon (Si), Argon (Ar)
4 9 Sulfur (S), Chromium (Cr), Iron (Fe), Germanium (Ge), Platinum (Pt), Lead (Pb)
5 ~10-11 Titanium (Ti), Nickel (Ni), Zinc (Zn), Molybdenum (Mo), Samarium (Sm), Mercury (Hg)
6 ~7-8 Krypton (Kr), Cadmium (Cd), Gadolinium (Gd), Hafnium (Hf), Osmium (Os)
7 ~4-5 Ruthenium (Ru), Dysprosium (Dy), Ytterbium (Yb)
8+ 2 Xenon (Xe) (9), Tin (Sn) (10)

Here is a re-categorization of the elements based on the natural prevalence (abundance) of their stable isotopes.

Elements by Predominant Number of Stable Isotopes

Overwhelmingly One Isotope (>90% Abundance)

This group includes elements that are either monoisotopic (100% abundance from one stable isotope) or have one isotope that is so dominant it accounts for over 90% of the element found in nature.

  • Monoisotopic (100%):
    • Beryllium (Be), Fluorine (F), Sodium (Na), Aluminium (Al), Phosphorus (P), Scandium (Sc), Manganese (Mn), Cobalt (Co), Arsenic (As), Yttrium (Y), Niobium (Nb), Rhodium (Rh), Iodine (I), Caesium (Cs), Praseodymium (Pr), Terbium (Tb), Holmium (Ho), Thulium (Tm), Gold (Au)
  • One Dominant Isotope (>90%):
    • Hydrogen (H): \(^{1}\)H at 99.98%
    • Helium (He): \(^{4}\)He at 99.999%
    • Carbon (C): \(^{12}\)C at 98.9%
    • Nitrogen (N): \(^{14}\)N at 99.6%
    • Oxygen (O): \(^{16}\)O at 99.76%
    • Neon (Ne): \(^{20}\)Ne at 90.48%
    • Silicon (Si): \(^{28}\)Si at 92.23%
    • Sulfur (S): \(^{32}\)S at 94.93%
    • Argon (Ar): \(^{40}\)Ar at 99.6%
    • Potassium (K): \(^{39}\)K at 93.26%
    • Calcium (Ca): \(^{40}\)Ca at 96.94%
    • Vanadium (V): \(^{51}\)V at 99.75%
    • Iron (Fe): \(^{56}\)Fe at 91.75%
    • Indium (In): \(^{115}\)In at 95.7% (Note: \(^{115}\)In is very slightly radioactive, but dominant)
    • Lanthanum (La): \(^{139}\)La at 99.91%
    • Cerium (Ce): \(^{140}\)Ce at 88.45% (Slightly under 90% but fits the spirit of this category)
    • Lutetium (Lu): \(^{175}\)Lu at 97.4%

Dominated by Two Isotopes

These elements have two stable isotopes that make up the vast majority of their natural abundance, with neither one being overwhelmingly dominant (i.e., neither is >90%). The ratios can be close to 50/50 or more skewed.

  • Lithium (Li): \(^{7}\)Li (92.5%) and \(^{6}\)Li (7.5%)
  • Boron (B): \(^{11}\)B (80.1%) and \(^{10}\)B (19.9%)
  • Chlorine (Cl): \(^{35}\)Cl (75.77%) and \(^{37}\)Cl (24.23%)
  • Copper (Cu): \(^{63}\)Cu (69.17%) and \(^{65}\)Cu (30.83%)
  • Gallium (Ga): \(^{69}\)Ga (60.11%) and \(^{71}\)Ga (39.89%)
  • Bromine (Br): \(^{79}\)Br (50.69%) and \(^{81}\)Br (49.31%)
  • Rubidium (Rb): \(^{85}\)Rb (72.17%) and \(^{87}\)Rb (27.83%)
  • Silver (Ag): \(^{107}\)Ag (51.84%) and \(^{109}\)Ag (48.16%)
  • Antimony (Sb): \(^{121}\)Sb (57.21%) and \(^{123}\)Sb (42.79%)
  • Europium (Eu): \(^{153}\)Eu (52.2%) and \(^{151}\)Eu (47.8%)
  • Iridium (Ir): \(^{193}\)Ir (62.7%) and \(^{191}\)Ir (37.3%)
  • Thallium (Tl): \(^{205}\)Tl (70.48%) and \(^{203}\)Tl (29.52%)

Dominated by Three Significant Isotopes

These elements have three isotopes with significant natural abundance, without a single one or pair being dominant.

  • Magnesium (Mg): \(^{24}\)Mg (78.99%), \(^{25}\)Mg (10.0%), \(^{26}\)Mg (11.01%)
  • Strontium (Sr): \(^{88}\)Sr (82.58%), \(^{86}\)Sr (9.86%), \(^{87}\)Sr (7.0%)
  • Lead (Pb): \(^{208}\)Pb (52.4%), \(^{206}\)Pb (24.1%), \(^{207}\)Pb (22.1%) (Note: \(^{204}\)Pb is 1.4%; all are stable end-products of decay chains).

Four or More Significant Isotopes

These elements have a complex mixture of four or more isotopes, each contributing a meaningful percentage to the total. This category includes the elements with the most diverse isotopic compositions.

  • Titanium (Ti): 5 stable isotopes, all >5% abundance
  • Chromium (Cr): 4 stable isotopes
  • Nickel (Ni): 5 stable isotopes
  • Zinc (Zn): 5 stable isotopes
  • Germanium (Ge): 5 stable isotopes
  • Selenium (Se): 6 stable isotopes
  • Krypton (Kr): 6 stable isotopes
  • Zirconium (Zr): 5 stable isotopes
  • Molybdenum (Mo): 7 stable isotopes
  • Ruthenium (Ru): 7 stable isotopes
  • Palladium (Pd): 6 stable isotopes
  • Cadmium (Cd): 8 stable isotopes
  • Tin (Sn): 10 stable isotopes (the most of any element)
  • Tellurium (Te): 8 stable isotopes
  • Xenon (Xe): 9 stable isotopes
  • Barium (Ba): 7 stable isotopes
  • Neodymium (Nd): 7 stable isotopes
  • Samarium (Sm): 7 stable isotopes
  • Gadolinium (Gd): 7 stable isotopes
  • Dysprosium (Dy): 7 stable isotopes
  • Erbium (Er): 6 stable isotopes
  • Ytterbium (Yb): 7 stable isotopes
  • Hafnium (Hf): 6 stable isotopes
  • Tungsten (W): 5 stable isotopes
  • Osmium (Os): 7 stable isotopes
  • Platinum (Pt): 6 stable isotopes
  • Mercury (Hg): 7 stable isotopes

Reversals

Reversals Among Elements with Stable Isotopes

These are the three classic examples taught in chemistry:

Element 1 (Lower Z) Atomic Number (Z) Standard Atomic Weight (u) Element 2 (Higher Z) Atomic Number (Z) Standard Atomic Weight (u)
Argon (Ar) 18 39.95 Potassium (K) 19 39.0983
Cobalt (Co) 27 58.9332 Nickel (Ni) 28 58.6934
Tellurium (Te) 52 127.60 Iodine (I) 53 126.9045

Reasoning:

  • Ar/K: Argon (18 protons) is dominated by the heavier \(^{40}\)Ar isotope (22 neutrons), which is a decay product of \(^{40}\)K. Potassium (19 protons) is overwhelmingly composed of the lighter \(^{39}\)K isotope (20 neutrons).
  • Co/Ni: Cobalt (27 protons) is 100% monoisotopic (\(^{59}\)Co). Nickel (28 protons) has a high abundance (68%) of the lighter \(^{58}\)Ni isotope, which pulls its average weight down.
  • Te/I: Tellurium (52 protons) has several heavy stable isotopes (\(^{128}\)Te and \(^{130}\)Te) that significantly raise its average weight. Iodine (53 protons) is 100% monoisotopic (\(^{127}\)I).

Reversals Involving Radioactive Elements

Reversals also appear among the heavy, radioactive elements. For these, the “atomic weight” is typically cited as the mass number of the longest-lived isotope (indicated by brackets) or, for primordial elements like Thorium and Uranium, a standard average weight.

Element 1 (Lower Z) Atomic Number (Z) Atomic Weight (u) Element 2 (Higher Z) Atomic Number (Z) Atomic Weight (u)
Thorium (Th) 90 232.0377 Protactinium (Pa) 91 231.0359
Uranium (U) 92 238.0289 Neptunium (Np) 93 [237]
Plutonium (Pu) 94 [244] Americium (Am) 95 [243]