89AcActinium227[Rn] 6d¹ 7s²
actinide actinium

Actinium

Actinium is a radioactive chemical element with symbol Ac (not to be confused with the abbreviation for an acetyl group) and atomic number 89, which was discovered in 1899. It was the first non-primordial radioactive element to be isolated. Polonium, radium and radon were observed before actinium, but they were not isolated until 1902.

Properties

At a glance

Atomic number
89
Atomic mass
227 u
Phase
Solid
Density
10 g/cm³
Melting point
1,500 K
Boiling point
3,500 K
First ionization energy
519.1 kJ/mol
Electronegativity
1.1
Electron affinity
33.77 kJ/mol
Molar heat capacity
27.2 J/(mol·K)
Period
7
Appearance
Unavailable

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

Electronic structure

Actinium’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…

89 protons138 neutrons

Actinium-227: 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, actinium 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

Ac

AcNeutral atom · no charge

A neutral actinium atom. Removing its first electron takes 519 kJ/mol.

Actinium’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 1.5× 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 7MeasuredEstimated 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). Actinium’s first electron takes about 5.4 eV.
1001,00010,000100,0001,000,00010,000,000100,000,000Shell 5Shell 4Shell 3×1.5×1.4×1.1×1.6×2.1×41510152025303540455055606570758085
Show the values as a table
ElectronShellEnergy (kJ/mol)× previousValue
17519–Measured
271,1342.2Estimated
361,6821.5Estimated
464,3232.6Estimated
565,3071.2Calculated
666,4651.2Calculated
767,6221.2Calculated
869,5421.3Calculated
9610,9901.2Calculated
10613,8841.3Calculated
11615,5441.1Calculated
12522,4811.4Calculated
13524,6041.1Calculated
14526,9191.1Calculated
15529,4281.1Calculated
16531,8401.1Calculated
17534,2521.1Calculated
18537,6291.1Calculated
19540,1381.1Calculated
20542,8391.1Calculated
21545,3481.1Calculated
22558,8561.3Calculated
23561,7511Calculated
24564,6451Calculated
25568,5051.1Calculated
26575,2591.1Calculated
27579,1181.1Calculated
28588,7671.1Calculated
29591,6611Calculated
30499,3801.1Calculated
314106,1341.1Calculated
324112,8881.1Calculated
334119,6421.1Calculated
344126,3961.1Calculated
354133,1501.1Calculated
364140,8691.1Calculated
374147,6231Calculated
384155,3411.1Calculated
394162,0951Calculated
404168,8491Calculated
414175,6031Calculated
424183,3221Calculated
434190,0761Calculated
444221,7231.2Calculated
454227,8981Calculated
464234,4591Calculated
474241,5031Calculated
484248,1601Calculated
494254,6251Calculated
504266,4921Calculated
514273,3431Calculated
524280,5791Calculated
534287,5261Calculated
544314,9281.1Calculated
554321,6821Calculated
564328,9181Calculated
574335,6721Calculated
584367,7061.1Calculated
594375,6171Calculated
604394,9141.1Calculated
614402,8261Estimated
623652,9161.6Calculated
633667,9681Calculated
643683,8881Calculated
653700,9661Calculated
663716,8861Calculated
673733,2891Calculated
683767,0581Calculated
693783,4611Calculated
703801,7931Calculated
713818,1961Calculated
723865,4731.1Calculated
733879,9461Calculated
743896,3491Calculated
753910,8221Calculated
7631,011,1661.1Calculated
7731,028,5341Calculated
7831,064,2331Calculated
7931,080,6361Calculated
8022,265,4762.1Calculated
8122,305,0351Calculated
8222,348,4531Calculated
8322,388,9771Calculated
8422,760,4451.2Calculated
8522,813,5121Calculated
8622,880,0871Calculated
8722,922,8401Calculated
88111,573,0014Calculated
89111,777,2991Calculated

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?

ActiniumGroup 3Other elements

Density

10 g/cm³27th highest of 99

Ac

Melting point

1,500 K37th highest of 96

Ac

Boiling point

3,500 K26th highest of 97

Ac

Electronegativity

1.111th lowest of 96

Ac

First ionization energy

519 kJ/mol8th lowest of 99

Ac

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.