Mononuclidic element

A mononuclidic element or monotopic element is one of the 21 chemical elements that is found naturally on Earth essentially as a single 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, and 2 in the second category. 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 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 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 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 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 and uncertainty. 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, the three most sensitive stable nuclei are hydrogen-1, fluorine-19 and phosphorus-31. 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, with a half-life of 1.387 million years, is produced by cosmic rays in the Earth's upper atmosphere; iodine-129, with a half-life of 16.1 million years, is produced by various cosmogenic and nuclear mechanisms; caesium-137, with a half-life of 30.04 years, is generated by nuclear fission. Such isotopes are used in a variety of analytical and forensic applications.

List of the 21 mononuclidic elements

Element	Most stable	Z	N	Isotopic mass	Half-life	Next most stable	N	Half-life
beryllium	⁹Be	4	5	9.012 183 06	Stable	¹⁰Be	6
fluorine	¹⁹F	9	10	18.998 403 1621	Stable	¹⁸F	9
sodium	²³Na	11	12	22.989 769 2820	Stable	²²Na	11
aluminium	²⁷Al	13	14	26.981 538 41	Stable	²⁶Al	13
phosphorus	³¹P	15	16	30.973 761 9977	Stable	³³P	18
scandium	⁴⁵Sc	21	24	44.955 907 1	Stable	⁴⁶Sc	25
manganese	⁵⁵Mn	25	30	54.938 043 04	Stable	⁵³Mn	28
cobalt	⁵⁹Co	27	32	58.933 193 5	Stable	⁶⁰Co	33
arsenic	⁷⁵As	33	42	74.921 594 6	Stable	⁷³As	40
yttrium	⁸⁹Y	39	50	88.905 838 2	Stable	⁸⁸Y	49
niobium	⁹³Nb	41	52	92.906 3732	Stable	⁹²Nb	51
rhodium	¹⁰³Rh	45	58	102.905 4941	Stable	¹⁰¹Rh	57
iodine	¹²⁷I	53	74	126.904 473	Stable	¹²⁹I	76
caesium	¹³³Cs	55	78	132.905 451 959	Stable	¹³⁵Cs	80
praseodymium	¹⁴¹Pr	59	82	140.907 6596	Stable	¹⁴³Pr	84
terbium	¹⁵⁹Tb	65	94	158.925 3537	Stable	¹⁵⁸Tb	93
holmium	¹⁶⁵Ho	67	98	164.930 329 1	Observationally stable	¹⁶³Ho	96
thulium	¹⁶⁹Tm	69	100	168.934 219 0	Observationally stable	¹⁷¹Tm	102
gold	¹⁹⁷Au	79	118	196.966 570 1	Observationally stable	¹⁹⁵Au	116
bismuth	²⁰⁹Bi	83	126	208.980 3986		^210mBi	127
protactinium	²³¹Pa	91	140	231.035 8825		²³³Pa	142