Mononuclidic element

File:Monoisotopic, mononuclidic, radioactive elements.svg

Template:Color strip Mononuclidic and monoisotopic (19 elements)
Template:Color strip Two mononuclidic, but radioactive elements (bismuth and protactinium)

A mononuclidic element or monotopic element is one of the 21 chemical elements that is found naturally on Earth essentially as a single nuclide (which may, or may not, be a stable nuclide). This single nuclide will have a characteristic atomic mass. Thus, the element's natural isotopic abundance is dominated by one isotope that is either stable or very long-lived. There are 19 elements in the first category (which are both monoisotopic and mononuclidic), and 2 (bismuthTemplate:Efn and protactinium) in the second category (mononuclidic but not monoisotopic, since they have zero, not one, stable nuclides). A list of the 21 mononuclidic elements is given at the end of this article.

Of the 26 monoisotopic elements that, by definition, have only one stable isotope, seven are not considered mononuclidic, due to the presence of a significant fraction of a very long-lived (primordial) radioisotope. These elements are vanadium, rubidium, indium, lanthanum, europium, lutetium, and rhenium.

Use in metrology

Many units of measurement were historically, or are still, defined with reference to the properties of specific substances that, in many cases, occurred in nature as mixes of multiple isotopes, for example:

Unit	Dimension	Reference substance	Relevant property	Number of common isotopes	Current (2022) status
Second	Time	Caesium	Hyperfine transition frequency	1	Still in use and one of the 7 SI base units
Metre	Length	Krypton	Transition wavelength	6	Redefined in 1983
Multiple	Temperature	Water	Melting point, boiling point, and triple point	2 of hydrogen and 3 of oxygen	Redefined in 2019 or defunct
Calorie and British thermal unit	Energy	Water	Specific heat capacity	2 of hydrogen and 3 of oxygen	Calorie redefined in terms of the joule, BTU still in use. Neither unit is part of, or recommended for use in, the SI
Mole	Amount of substance	Carbon	Atomic mass	3	Redefined in 2019
Dalton	Mass	Carbon	Atomic mass	3	Still in use and accepted for use in (but not part of) the SI
Candela	Luminous intensity	Platinum	Luminance at melting point	6	Redefined in 1979
Millimetre of mercury	Pressure	Mercury	Density	7	Redefined in terms of the pascal, not part of, or recommended for use in, the SI

Since samples taken from different natural sources can have subtly different isotopic ratios, the relevant properties can differ between samples. If the definition simply refers to a substance without addressing the isotopic composition, this can lead to some level of ambiguity in the definition and variation in practical realizations of the unit by different laboratories, as was observed with the kelvin before 2007. If the definition refers only to one isotope (as that of the dalton does) or to a specific isotope ratio, e.g. Vienna Standard Mean Ocean Water, this removes a source of ambiguity and variation, but adds layers of technical difficulty (preparing samples of a desired isotopic ratio) and uncertainty (regarding how much an actual reference sample differs from the nominal ratio). The use of mononuclidic elements as reference material sidesteps these issues and notably the only substance referenced in the most recent iteration of the SI is caesium, a mononuclidic element.

Mononuclidic elements are also of scientific importance because their atomic weights can be measured to high accuracy, since there is minimal uncertainty associated with the isotopic abundances present in a given sample. Another way of stating this, is that, for these elements, the standard atomic weight and atomic mass are the same.

In practice, only 11 of the mononuclidic elements are used in standard atomic weight metrology. These are aluminium, bismuth, caesium, cobalt, gold, manganese, phosphorus, scandium, sodium, terbium, and thorium.

In nuclear magnetic resonance spectroscopy (NMR), the three most sensitive stable nuclei are hydrogen-1 (¹H), fluorine-19 (¹⁹F) and phosphorus-31 (³¹P). Fluorine and phosphorus are monoisotopic, with hydrogen nearly so. ¹H NMR, ¹⁹F NMR and ³¹P NMR allow for identification and study of compounds containing these elements.

Contamination by unstable trace isotopes

Trace concentrations of unstable isotopes of some mononuclidic elements are found in natural samples. For example, beryllium-10 (¹⁰Be), with a half-life of 1.4 million years, is produced by cosmic rays in the Earth's upper atmosphere; iodine-129 (¹²⁹I), with a half-life of 15.7 million years, is produced by various cosmogenic and nuclear mechanisms; caesium-137 (¹³⁷Cs), with a half-life of 30 years, is generated by nuclear fission. Such isotopes are used in a variety of analytical and forensic applications.

List of the 21 mononuclidic elements

Isotopic mass data from Atomic Weights and Isotopic Compositions ed. J. S. Coursey, D. J. Schwab and R. A. Dragoset, National Institute of Standards and Technology (2005).

Element	Most stable nuclide	Z (p)	N (n)	Isotopic mass (Da)	Half-life	Second most stable nuclide	N (n)	Half-life
beryllium	9Be	4	5	9.012 182(3)	Stable	10Be	6
fluorine	19F	9	10	18.998 403 2(5)	Stable	18F	9
sodium	23Na	11	12	22.989 770(2)	Stable	22Na	11
aluminium	27Al	13	14	26.981 538(2)	Stable	26Al	13
phosphorus	31P	15	16	30.973 761(2)	Stable	33P	18
scandium	45Sc	21	24	44.955 910(8)	Stable	46Sc	25
manganese	55Mn	25	30	54.938 049(9)	Stable	53Mn	28
cobalt	59Co	27	32	58.933 200(9)	Stable	60Co	33
arsenic	75As	33	42	74.921 60(2)	Stable	73As	40
yttrium	89Y	39	50	88.905 85(2)	Stable	88Y	49
niobium	93Nb	41	52	92.906 38(2)	Stable	92Nb	51
rhodium	103Rh	45	58	102.905 50(2)	Stable	102mRh	57
iodine	127I	53	74	126.904 47(3)	Stable	129I	76
caesium	133Cs	55	78	132.905 45(2)	Stable	135Cs	80
praseodymium	141Pr	59	82	140.907 65(2)	Stable	143Pr	84
terbium	159Tb	65	94	158.925 34(2)	Stable	158Tb	93
holmium	165Ho	67	98	164.930 32(2)	Observationally stable	163Ho	96
thulium	169Tm	69	100	168.934 21(2)	Observationally stable	171Tm	102
gold	197Au	79	118	196.966 55(2)	Observationally stable	195Au	116
bismuth	209Bi	83	126	208.980 38(2)		210mBi	127
protactinium	231Pa	91	140	231.035 88(2)		233Pa	142

See also

• Primordial element
• Table of nuclides sorted by half-life
• Table of nuclides
• Isotope geochemistry
• Radionuclide
• List of elements by stability of isotopes