61PmPromethium145[Xe] 4f⁵ 6s²
lanthanide

Promethium

Promethium, originally prometheum, is a chemical element with the symbol Pm and atomic number 61. All of its isotopes are radioactive; it is one of only two such elements that are followed in the periodic table by elements with stable forms, a distinction shared with technetium. Chemically, promethium is a lanthanide, which forms salts when combined with other elements.

Properties

At a glance

Atomic number
61
Atomic mass
145 u
Phase
Solid
Density
7.26 g/cm³
Melting point
1,315 K
Boiling point
3,273 K
First ionization energy
538.6 kJ/mol
Electronegativity
1.13
Electron affinity
12.45 kJ/mol
Molar heat capacity
Unavailable
Period
6
Appearance
metallic

Conditions and isotopes affect measured properties. Some properties of short-lived elements are predicted. Electronegativity uses the Pauling scale.

Electronic structure

Promethium’s orbitals use a hydrogen-like approximation. Every orbital of each visible subshell is shown: in Shapes as the surface holding half its probability, with light and dark shades for opposite signs of the wavefunction; in Cloud as dots where its electrons are likely to be. Sizes are stepped by shell, like the Shell model.

Electrons by shell
Electron configuration

54 is the [Xe] core total across five shells.

Preparing the orbital view…

61 protons84 neutrons

Promethium-145: its most common form in nature, or its longest-lived if it has none. The nucleus is drawn far too big: it’s about 100,000 times smaller than the atom.

States of matter

Turn up the heat

298 KK stands for kelvin, the temperature scale used in science. It starts at absolute zero (0 K), the coldest temperature possible, so it has no negative numbers and no degree sign.

A step of 1 K is the same size as 1 °C, so the two scales are always 273.15 apart. Fahrenheit is different: its degrees are smaller and it starts elsewhere, so water freezes at 273.15 K, 0 °C and 32 °F.25 °C · 77 °FSolid

At this temperature, promethium is a solid.

At normal atmospheric pressure (1 atm). The temperature scale is logarithmic. The particle box is schematic: real samples hold vastly more atoms, and spacing and speeds are not to scale.

Ionization energy

Pull the electrons off

Pm

PmNeutral atom · no charge

A neutral promethium atom. Removing its first electron takes 539 kJ/mol.

Promethium’s outer shell holds 2 electrons. Electron 3 is the first from an inner shell: it sits closer to the nucleus with less shielding, so removing it takes 2.1× as much energy as the one before. Each dashed line on the chart marks another jump, into a shell even closer to the nucleus.

Shell 1Shell 2Shell 3Shell 4Shell 5Shell 6MeasuredEstimated or calculatedEnergy in kJ/molkJ/mol (kilojoules per mole) is the energy to remove that electron from every atom in a mole of them: about 602 sextillion (6.02 × 10²³) atoms. For a single atom, divide by 96.485 to get electronvolts (eV). Promethium’s first electron takes about 5.6 eV.
1001,00010,000100,0001,000,00010,000,000Shell 5Shell 4Shell 3Shell 2×2.1×1.1×1.7×2.3×4.1151015202530354045505560
Show the values as a table
ElectronShellEnergy (kJ/mol)× previousValue
16539–Measured
261,0552Estimated
352,1652.1Estimated
453,9721.8Estimated
555,9531.5Estimated
658,2011.4Calculated
759,7451.2Calculated
8511,1921.1Calculated
9513,3151.2Calculated
10514,9551.1Calculated
11416,7881.1Calculated
12419,4901.2Calculated
13422,0951.1Calculated
14423,9281.1Calculated
15425,9551.1Calculated
16441,4891.6Calculated
17444,5761.1Calculated
18447,9531.1Calculated
19451,5231.1Calculated
20454,9001.1Calculated
21458,7601.1Calculated
22462,8121.1Calculated
23466,4781.1Calculated
24470,4341.1Calculated
25474,0041.1Calculated
26488,3811.2Calculated
27492,2401Calculated
28496,2921Calculated
294100,3451Calculated
304107,3881.1Calculated
314111,7301Calculated
324121,6681.1Calculated
334126,2701Estimated
343217,1881.7Calculated
353226,1621Calculated
363235,7141Calculated
373245,9411Calculated
383255,8791Calculated
393265,8171Calculated
403279,0361Calculated
413289,1671Calculated
423300,2621Calculated
433310,5861Calculated
443339,5321.1Calculated
453348,6021Calculated
463358,7321Calculated
473368,1881Calculated
483391,3451.1Calculated
493401,9581Calculated
503421,7371Calculated
513431,8681Calculated
522975,9492.3Calculated
5321,001,3251Calculated
5421,029,5951Calculated
5521,055,7431Calculated
5621,140,3601.1Calculated
5721,170,9461Calculated
5821,209,1541Calculated
5921,234,7481Calculated
6015,031,1594.1Calculated
6115,147,1361Calculated

Successive ionization energies of gas-phase atoms and ions from the NIST Atomic Spectra Database. Solid bars are measured; hatched bars were estimated from nearby measurements or calculated from quantum theory, as is usual for highly charged ions. Electrons are taken from the outermost shell first, a simplification: electrons in different subshells of the same shell also differ in energy. On the log scale each gridline is 10 times the one below.

Compared with other elements

How does it compare?

PromethiumOther lanthanidesOther elements

Density

7.26 g/cm³47th highest of 99

Pm

Melting point

1,315 K43rd highest of 96

Pm

Boiling point

3,273 K31st highest of 97

Pm

Electronegativity

1.1317th lowest of 96

Pm

First ionization energy

539 kJ/mol15th lowest of 99

Pm

Elements 1–99 from the reference data; heavier elements are left out because only a few atoms have ever been made, so their bulk properties are predictions. First ionization energies come from NIST. Density uses a logarithmic scale, because gases are around a thousand times less dense than solids; gas densities are at 0 °C and 1 atm. Melting and boiling points are at normal pressure. Everyday temperatures and light sources in the pictures are approximate. Hover over a dot to see the element, or click it to compare.

Keep exploring

Explore the stories, imagery, and reference data in more depth. Properties with no value in the reference are marked unavailable.