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The current evaluated value for promethium’s first ionization energy is 5.58187 ± 0.00004 eV, according to the NIST Atomic Spectra Database version 5.12. In a periodic-table format, NIST rounds it to 5.5820 eV. Both figures describe the same process: removing the first electron from a neutral, ground-state promethium atom in the gas phase.
The value to use today
If you need one authoritative number for the first ionization energy of promethium (Pm), use 5.58187 ± 0.00004 eV. This is the evaluated entry in the National Institute of Standards and Technology (NIST) Atomic Spectra Database, version 5.12, with an output reference date of 1 October 2026. The uncertainty belongs to the reported value; it is not an indication that the energy changes from atom to atom under the same definition.
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| Source and date | Displayed value | How to interpret it |
|---|---|---|
| NIST Atomic Spectra Database, version 5.12 (2024; output reference date 2026-10-01) | 5.58187 ± 0.00004 eV | Current evaluated first ionization energy for neutral ground-state Pm |
| NIST periodic table, June 2024 | 5.5820 eV | Rounded periodic-table presentation of the same quantity |
| NIST Atomic Data for Promethium | 45020 cm−1 (5.582 eV) | Compiled value shown with a wavenumber representation and rounded energy |
The extra digits in the Atomic Spectra Database entry and the four-decimal display in the periodic table are different presentations, not competing modern measurements. For ordinary prose, about 5.582 eV is an appropriate rounded statement. Use the full 5.58187 ± 0.00004 eV form when precision and uncertainty matter.
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First ionization energy is the energy required to remove one electron from a neutral atom in the gas phase. “First” matters: the starting species is Pm with no net charge, and the product is Pm+ after one electron has been removed. Energies for taking additional electrons from Pm+ or from later ions are successive ionization energies and are not alternatives to the value above.
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NIST’s entry refers specifically to neutral promethium in its ground state. Its listed electron configuration is [Xe]4f56s2, and its ground level is written 6H°5/2. Those labels identify the electronic state used to define the threshold. A value attached to an excited atom, a molecular species, a condensed sample, or an already charged ion would answer a different question.
Why the gas-phase qualification is essential
Ionization energy tables describe isolated atoms in the gas phase. In a solid, liquid, compound, or crystal, surrounding atoms and chemical bonds alter electron energies. A measured work function or a compound’s ionization behavior therefore cannot be substituted for the atomic first-ionization energy. The NIST number is an atomic property under the gas-phase definition.
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Why older tables show a different number
Historical compilations commonly place promethium near 5.55 eV, while modern NIST displays 5.582 eV. The difference reflects the particular determination, its uncertainty, and whether a later database evaluation or an original experiment is being quoted. It does not mean that promethium has several valid first-ionization energies.
| Historical source | Reported value | Context |
|---|---|---|
| Worden, Solarz, Paisner and Conway, 1978, as listed by the NIST Chemistry WebBook | 5.582 ± 0.010 eV | Laser-spectroscopy determination recorded in the WebBook’s historical data |
| National Bureau of Standards compilation, 1960 | 5.554 ± 0.020 eV | Older compilation summarizing an earlier determination |
The 1978 result overlaps the modern value within its substantially larger uncertainty. The 1960 compilation has a central value lower than today’s evaluated result and an uncertainty of 0.020 eV. When a table prints only “5.55 eV,” it may be shortening that older determination and hiding the uncertainty that explains why it should not be compared digit-for-digit with the NIST ASD entry.
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NIST’s modern database is an evaluated reference entry, whereas a historical paper or compilation records what was obtained with the methods and calibration available at that time. The available records establish the differing reported values and uncertainties; they do not by themselves justify a specific story about why NIST changed the recommended number. The safe comparison is to identify the source, date, precision, uncertainty, and whether the figure is historical or evaluated.
How to read the precision
The uncertainty is part of the result
In 5.58187 ± 0.00004 eV, 5.58187 eV is the central evaluated value and 0.00004 eV is the stated uncertainty. Reporting all six decimal places without the uncertainty would overstate what the entry communicates. Conversely, replacing the value with 5.58 eV is acceptable only when a low-precision summary is sufficient.
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Rounded tables are not extra measurements
NIST’s June 2024 periodic table prints 5.5820 eV. The trailing zero records the table’s chosen display precision; it does not add a new experimental result. A poster or classroom table may use still fewer digits. Digits shown across a periodic table should not be assumed to have identical evidential precision for every element, because each element’s value can come from different measurements and evaluations.
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Unit choices
The NIST Atomic Data for Promethium compilation also gives 45020 cm−1, corresponding to a rounded 5.582 eV. Wavenumber and electron-volt forms are two units for the same threshold. Keep the unit attached when copying a number, and do not combine the wavenumber precision from one display with the electron-volt uncertainty from another as though they were independent measurements.
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Promethium’s electronic state behind the number
The configuration [Xe]4f56s2 describes the electrons of neutral ground-state promethium beyond the xenon core. The 4f and 6s occupations, together with the ground-level designation 6H°5/2, specify the state from which the first electron is removed. This is why a database entry can be more precise than a bare element-symbol lookup: it identifies the atom and the state, not just the chemical name.
Promethium is unusual among the lanthanides because all known isotopes are relatively short-lived, as noted in the National Bureau of Standards rare-earth compilation. That practical scarcity helps explain why authoritative atomic data are especially valuable: the reference quantity is established through spectroscopic and compiled atomic measurements rather than routine handling of a stable bulk specimen. The short half-lives do not change the definition of first ionization energy, but they are relevant to how such data are obtained and preserved.
Choosing the right number for a citation or calculation
- For a technical citation: quote 5.58187 ± 0.00004 eV and identify the NIST Atomic Spectra Database, version 5.12. Include that it applies to neutral, ground-state promethium.
- For a general explanation: write “promethium’s first ionization energy is about 5.582 eV.” This keeps the rounded value readable without implying more precision than the context needs.
- For a periodic-table transcription: use NIST’s June 2024 display, 5.5820 eV, and label it as the rounded table value.
- For historical analysis: preserve the original uncertainty and date, such as 5.582 ± 0.010 eV (1978) or 5.554 ± 0.020 eV (1960). Do not silently replace a historical number with the modern evaluation.
Common mistakes to avoid
- Confusing first and successive ionization: the title’s quantity starts with neutral Pm. Energies for Pm+ and higher ions belong to different entries.
- Dropping “gas phase”: a solid’s work function or a compound’s ionization behavior is not the atomic first-ionization energy.
- Treating 5.5820 eV as a separate experiment: it is the rounded periodic-table presentation of the same evaluated quantity.
- Mixing historical and current precision: 5.554 ± 0.020 eV from a 1960 compilation cannot be compared with 5.58187 ± 0.00004 eV without carrying both dates and uncertainties.
- Reporting unsupported digits: do not append more decimal places than the cited source provides, and do not omit the uncertainty when the precision is central to the discussion.
- Assuming every table is current: check the database version or publication date before deciding which value to quote.
A concise timeline of the reported values
The 1960 National Bureau of Standards compilation records an older determination as 5.554 ± 0.020 eV. A 1978 laser-spectroscopy result, preserved in the NIST Chemistry WebBook, gives 5.582 ± 0.010 eV. NIST’s evaluated Atomic Spectra Database entry now reports 5.58187 ± 0.00004 eV, while the June 2024 periodic table rounds that result to 5.5820 eV. Read in sequence, these entries show increasing refinement and different display conventions rather than a changing physical constant.
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Bottom line for readers
Use 5.58187 ± 0.00004 eV as the current NIST value for the first ionization energy of neutral, ground-state promethium. Use 5.5820 eV when reproducing NIST’s June 2024 periodic-table format, and identify values near 5.55 eV as historical determinations with larger uncertainties. The defining atom is gas-phase Pm with configuration [Xe]4f56s2; later ionization stages are separate quantities.
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