91PaProtactinium231.035882[Rn] 5f² 6d¹ 7s²
actinide

Protactinium

Protactinium is a chemical element with symbol Pa and atomic number 91. It is a dense, silvery-gray metal which readily reacts with oxygen, water vapor and inorganic acids. It forms various chemical compounds where protactinium is usually present in the oxidation state +5, but can also assume +4 and even +2 or +3 states.

Properties

At a glance

Atomic number
91
Atomic mass
231.035882 u
Phase
Solid
Density
15.37 g/cm³
Melting point
1,841 K
Boiling point
4,300 K
First ionization energy
568.3 kJ/mol (estimated)
Electronegativity
1.5
Electron affinity
53.03 kJ/mol
Molar heat capacity
Unavailable
Period
7
Appearance
bright, silvery metallic luster

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

Electronic structure

Protactinium’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

86 is the [Rn] core total across six shells.

Preparing the orbital view…

91 protons140 neutrons

Protactinium-231: 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, protactinium 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

Pa

PaNeutral atom · no charge

A neutral protactinium atom. Removing its first electron takes 568 kJ/mol.

Protactinium’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 1.6× 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 6Shell 7Estimated 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). Protactinium’s first electron takes about 5.9 eV.
1001,00010,000100,0001,000,00010,000,000100,000,000Shell 5Shell 4Shell 3×1.6×1.2×1.1×1.6×2.1×4151015202530354045505560657075808590
Show the values as a table
ElectronShellEnergy (kJ/mol)× previousValue
17568–Estimated
271,1482Estimated
361,7951.6Estimated
462,9811.7Estimated
564,2741.4Calculated
666,9471.6Calculated
768,2111.2Calculated
869,5421.2Calculated
9610,7101.1Calculated
10613,2181.2Calculated
11614,7621.1Calculated
12518,0431.2Calculated
13519,5871.1Calculated
14528,1741.4Calculated
15530,4891.1Calculated
16532,9981.1Calculated
17535,6031.1Calculated
18538,1121.1Calculated
19540,8131.1Calculated
20544,3831.1Calculated
21547,0851.1Calculated
22549,9791.1Calculated
23552,6811.1Calculated
24566,5751.3Calculated
25569,4691Calculated
26573,3291.1Calculated
27576,2231Calculated
28584,9071.1Calculated
29588,7671Calculated
30598,4151.1Calculated
315102,2741Calculated
324110,9581.1Calculated
334117,7121.1Calculated
344125,4311.1Calculated
354132,1851.1Calculated
364139,9041.1Calculated
374146,6581Calculated
384154,3771.1Calculated
394161,1311Calculated
404169,8141.1Calculated
414176,5681Calculated
424184,2871Calculated
434191,0411Calculated
444198,7601Calculated
454205,5141Calculated
464239,5731.2Calculated
474246,0381Calculated
484252,7921Calculated
494260,1241Calculated
504266,8781Calculated
514273,7291Calculated
524286,3681Calculated
534293,3151Calculated
544300,9381Calculated
554308,0781Calculated
564336,5411.1Calculated
574343,2951Calculated
584350,9171Calculated
594357,8641Calculated
604393,3711.1Calculated
614401,4751Calculated
624421,6411.1Calculated
634429,7461Estimated
643692,8611.6Calculated
653708,2991Calculated
663724,6051Calculated
673741,9721Calculated
683759,3401Calculated
693775,7421Calculated
703811,4421Calculated
713828,8091Calculated
723847,1411Calculated
733864,5091Calculated
743912,7511.1Calculated
753928,1891Calculated
763944,5911Calculated
773960,0291Calculated
7831,070,9871.1Calculated
7931,089,3191Calculated
8031,125,0191Calculated
8131,142,3861Calculated
8222,379,3282.1Calculated
8322,419,8521Calculated
8422,464,2351Calculated
8522,505,7241Calculated
8622,916,7521.2Calculated
8722,971,7481Calculated
8823,041,2181Calculated
8923,084,7901Calculated
90112,186,5614Calculated
91112,399,0411Calculated

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?

ProtactiniumOther actinidesOther elements

Density

15.4 g/cm³11th highest of 99

Pa

Melting point

1,841 K20th highest of 96

Pa

Boiling point

4,300 K15th highest of 97

Pa

Electronegativity

1.541st lowest of 96

Pa

First ionization energy

568 kJ/mol21st lowest of 99

Pa

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.