Radium
Radium is a chemical element with symbol Ra and atomic number 88. It is the sixth element in group 2 of the periodic table, also known as the alkaline earth metals. Pure radium is almost colorless, but it readily combines with nitrogen (rather than oxygen) on exposure to air, forming a black surface layer of radium nitride (Ra3N2).
At a glance
- Atomic number
- 88
- Atomic mass
- 226 u
- Phase
- Solid
- Density
- 5.5 g/cm³
- Melting point
- 1,233 K
- Boiling point
- 2,010 K
- First ionization energy
- 509.3 kJ/mol
- Electronegativity
- 0.9
- Electron affinity
- 9.6485 kJ/mol
- Molar heat capacity
- Unavailable
- Period
- 7
- Appearance
- silvery white metallic
Conditions and isotopes affect measured properties. Some properties of short-lived elements are predicted. Electronegativity uses the Pauling scale.
Radium’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.
86 is the [Rn] core total across six shells.
Preparing the orbital view…
88 protons138 neutrons
Radium-226: 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.
Turn up the heat
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, radium 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.
Pull the electrons off
RaNeutral atom · no charge
A neutral radium atom. Removing its first electron takes 509 kJ/mol.
Radium’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 3.1× as much energy as the one before. That’s why radium forms Ra²⁺ ions. Each dashed line on the chart marks another jump, into a shell even closer to the nucleus.
What does this tell us?
- The 2 electrons before the first big jump are radium’s valence electrons, the ones it uses in chemical reactions. That’s why radium sits in group 2: every element in the group has the same number of outer electrons, so they react in similar ways.
- The numbers explain the charge. Removing electron 3 as well, to make Ra³⁺, would cost 2,991 kJ/mol. Forming a compound gives back far less energy than that, so Ra³⁺ doesn’t form in ordinary chemistry.
- Within a shell the steps rise gently. Each electron removed leaves fewer electrons pushing the others away, while the nucleus pulls just as hard, so the next one is held a little more tightly.
Show the values as a table
| Electron | Shell | Energy (kJ/mol) | × previous | Value |
|---|---|---|---|---|
| 1 | 7 | 509 | – | Measured |
| 2 | 7 | 979 | 1.9 | Measured |
| 3 | 6 | 2,991 | 3.1 | Calculated |
| 4 | 6 | 3,956 | 1.3 | Calculated |
| 5 | 6 | 5,104 | 1.3 | Calculated |
| 6 | 6 | 6,175 | 1.2 | Calculated |
| 7 | 6 | 7,912 | 1.3 | Calculated |
| 8 | 6 | 9,359 | 1.2 | Calculated |
| 9 | 6 | 11,964 | 1.3 | Calculated |
| 10 | 6 | 13,508 | 1.1 | Calculated |
| 11 | 5 | 19,770 | 1.5 | Calculated |
| 12 | 5 | 21,902 | 1.1 | Calculated |
| 13 | 5 | 24,121 | 1.1 | Calculated |
| 14 | 5 | 26,437 | 1.1 | Calculated |
| 15 | 5 | 28,849 | 1.1 | Calculated |
| 16 | 5 | 31,261 | 1.1 | Calculated |
| 17 | 5 | 34,349 | 1.1 | Calculated |
| 18 | 5 | 36,857 | 1.1 | Calculated |
| 19 | 5 | 39,463 | 1.1 | Calculated |
| 20 | 5 | 41,971 | 1.1 | Calculated |
| 21 | 5 | 54,997 | 1.3 | Calculated |
| 22 | 5 | 57,891 | 1.1 | Calculated |
| 23 | 5 | 60,786 | 1.1 | Calculated |
| 24 | 5 | 63,680 | 1 | Calculated |
| 25 | 5 | 71,399 | 1.1 | Calculated |
| 26 | 5 | 74,294 | 1 | Calculated |
| 27 | 5 | 82,977 | 1.1 | Calculated |
| 28 | 5 | 86,837 | 1 | Calculated |
| 29 | 4 | 93,591 | 1.1 | Calculated |
| 30 | 4 | 100,345 | 1.1 | Calculated |
| 31 | 4 | 107,099 | 1.1 | Calculated |
| 32 | 4 | 113,853 | 1.1 | Calculated |
| 33 | 4 | 120,607 | 1.1 | Calculated |
| 34 | 4 | 127,361 | 1.1 | Calculated |
| 35 | 4 | 134,115 | 1.1 | Calculated |
| 36 | 4 | 140,869 | 1.1 | Calculated |
| 37 | 4 | 147,623 | 1 | Calculated |
| 38 | 4 | 155,341 | 1.1 | Calculated |
| 39 | 4 | 162,095 | 1 | Calculated |
| 40 | 4 | 168,849 | 1 | Calculated |
| 41 | 4 | 175,603 | 1 | Calculated |
| 42 | 4 | 181,392 | 1 | Calculated |
| 43 | 4 | 213,040 | 1.2 | Calculated |
| 44 | 4 | 219,118 | 1 | Calculated |
| 45 | 4 | 225,583 | 1 | Calculated |
| 46 | 4 | 232,433 | 1 | Calculated |
| 47 | 4 | 238,994 | 1 | Calculated |
| 48 | 4 | 245,459 | 1 | Calculated |
| 49 | 4 | 256,844 | 1 | Calculated |
| 50 | 4 | 263,501 | 1 | Calculated |
| 51 | 4 | 270,738 | 1 | Calculated |
| 52 | 4 | 277,492 | 1 | Calculated |
| 53 | 4 | 304,411 | 1.1 | Calculated |
| 54 | 4 | 311,069 | 1 | Calculated |
| 55 | 4 | 318,209 | 1 | Calculated |
| 56 | 4 | 324,963 | 1 | Calculated |
| 57 | 4 | 355,259 | 1.1 | Calculated |
| 58 | 4 | 362,978 | 1 | Calculated |
| 59 | 4 | 381,985 | 1.1 | Calculated |
| 60 | 4 | 389,801 | 1 | Estimated |
| 61 | 3 | 633,426 | 1.6 | Calculated |
| 62 | 3 | 648,188 | 1 | Calculated |
| 63 | 3 | 663,916 | 1 | Calculated |
| 64 | 3 | 680,801 | 1 | Calculated |
| 65 | 3 | 696,817 | 1 | Calculated |
| 66 | 3 | 712,062 | 1 | Calculated |
| 67 | 3 | 744,867 | 1 | Calculated |
| 68 | 3 | 761,269 | 1 | Calculated |
| 69 | 3 | 779,601 | 1 | Calculated |
| 70 | 3 | 796,004 | 1 | Calculated |
| 71 | 3 | 842,317 | 1.1 | Calculated |
| 72 | 3 | 856,790 | 1 | Calculated |
| 73 | 3 | 872,227 | 1 | Calculated |
| 74 | 3 | 887,665 | 1 | Calculated |
| 75 | 3 | 983,186 | 1.1 | Calculated |
| 76 | 3 | 999,588 | 1 | Calculated |
| 77 | 3 | 1,034,323 | 1 | Calculated |
| 78 | 3 | 1,050,725 | 1 | Calculated |
| 79 | 2 | 2,209,514 | 2.1 | Calculated |
| 80 | 2 | 2,248,108 | 1 | Calculated |
| 81 | 2 | 2,291,527 | 1 | Calculated |
| 82 | 2 | 2,331,086 | 1 | Calculated |
| 83 | 2 | 2,685,187 | 1.2 | Calculated |
| 84 | 2 | 2,737,289 | 1 | Calculated |
| 85 | 2 | 2,802,899 | 1 | Calculated |
| 86 | 2 | 2,844,368 | 1 | Calculated |
| 87 | 1 | 11,274,649 | 4 | Calculated |
| 88 | 1 | 11,474,952 | 1 | Calculated |
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.
Watching atoms decay
Marie and Pierre Curie discovered radium in 1898, in uranium ore. It is there because radium-226 is one stop on uranium-238’s decay chain, constantly being made and destroyed.
Half of any sample of Ra-226 decays every 1,600 years, turning into Rn-222.
Half-lives passed (1 half-life = 1,600 years)
No time passed · 0 half-lives · 100 of 100 atoms left
Each atom decays at a random moment, so no one can say which will go next, but about half of whatever is left goes in every half-life. The dashed curve is the prediction; the solid line is this sample. The new Rn-222 atoms decay too, on their own schedule, which the chain below follows.
From U-238 to stable Pb-206: 14 steps
- U-238Uranium4.5 billion yearsαdecays by alpha
- Th-234Thorium24 daysβdecays by beta
- Pa-234mProtactinium1.2 minutesβdecays by beta
- U-234Uranium250,000 yearsαdecays by alpha
- Th-230Thorium75,000 yearsαdecays by alpha
- Ra-226Radium1,600 yearsαdecays by alpha
- Rn-222Radon3.8 daysαdecays by alpha
- Po-218Polonium3.1 minutesαdecays by alpha
- Pb-214Lead27 minutesβdecays by beta
- Bi-214Bismuth20 minutesβdecays by beta
- Po-214Polonium160 microsecondsαdecays by alpha
- Pb-210Lead22 yearsβdecays by beta
- Bi-210Bismuth5 daysβdecays by beta
- Po-210Polonium140 daysαdecays by alpha
- Pb-206Leadstable
Arrows show how each atom decays: α gives off an alpha particle (2 protons and 2 neutrons), β turns a neutron into a proton, γ gives off a gamma ray.
Half-lives and decay modes from the IAEA Live Chart of Nuclides (evaluated ENSDF data). The chain follows each atom’s most common decay; a few steps have rare side branches.
How does it compare?
Density
5.5 g/cm³37th lowest of 99
Melting point
1,233 K47th highest of 96
Boiling point
2,010 K39th lowest of 97
Electronegativity
0.96th lowest of 96
First ionization energy
509 kJ/mol7th lowest of 99
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.
- RadiumRoyal Society of Chemistry
- RadiumWikipedia
- Quantum numbers and electron configurationsPurdue
- Hydrogenic wavefunctionsNIST
- Ionization energiesNIST Atomic Spectra Database