Thallium
Thallium is a chemical element with symbol Tl and atomic number 81. This soft gray post-transition metal is not found free in nature. When isolated, it resembles tin, but discolors when exposed to air.
At a glance
- Atomic number
- 81
- Atomic mass
- 204.38 u
- Phase
- Solid
- Density
- 11.85 g/cm³
- Melting point
- 577 K
- Boiling point
- 1,746 K
- First ionization energy
- 589.4 kJ/mol
- Electronegativity
- 1.62
- Electron affinity
- 36.4 kJ/mol
- Molar heat capacity
- 26.32 J/(mol·K)
- Period
- 6
- Appearance
- silvery white
Conditions and isotopes affect measured properties. Some properties of short-lived elements are predicted. Electronegativity uses the Pauling scale.
Thallium’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.
54 is the [Xe] core total across five shells.
Preparing the orbital view…
81 protons124 neutrons
Thallium-205: 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, thallium 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
TlNeutral atom · no charge
A neutral thallium atom. Removing its first electron takes 589 kJ/mol.
Thallium’s outer shell holds 3 electrons. Even inside it there’s a jump: electron 2 takes 3.3× as much as electron 1, because it sits in a lower-energy subshell. Electron 4 is the first from an inner shell: it sits closer to the nucleus with less shielding, so removing it takes 1.7× as much energy as the one before. Each dashed line on the chart marks another jump, into a shell even closer to the nucleus.
What does this tell us?
- The 3 electrons before the first big jump are thallium’s valence electrons, the ones it uses in chemical reactions. That’s why thallium sits in group 13: every element in the group has the same number of outer electrons, so they react in similar ways.
- 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.
- The jump inside the outer shell is evidence that shells are split into subshells (s, p, d and f), each with its own energy. You can explore their shapes in the Orbitals view above.
Show the values as a table
| Electron | Shell | Energy (kJ/mol) | × previous | Value |
|---|---|---|---|---|
| 1 | 6 | 589 | – | Measured |
| 2 | 6 | 1,971 | 3.3 | Measured |
| 3 | 6 | 2,880 | 1.5 | Measured |
| 4 | 5 | 4,934 | 1.7 | Measured |
| 5 | 5 | 6,040 | 1.2 | Measured |
| 6 | 5 | 7,719 | 1.3 | Calculated |
| 7 | 5 | 9,446 | 1.2 | Calculated |
| 8 | 5 | 11,192 | 1.2 | Calculated |
| 9 | 5 | 13,026 | 1.2 | Calculated |
| 10 | 5 | 15,245 | 1.2 | Calculated |
| 11 | 5 | 17,078 | 1.1 | Calculated |
| 12 | 5 | 19,104 | 1.1 | Calculated |
| 13 | 5 | 21,063 | 1.1 | Estimated |
| 14 | 5 | 29,611 | 1.4 | Estimated |
| 15 | 5 | 32,805 | 1.1 | Calculated |
| 16 | 5 | 35,314 | 1.1 | Calculated |
| 17 | 5 | 37,822 | 1.1 | Calculated |
| 18 | 5 | 42,357 | 1.1 | Calculated |
| 19 | 5 | 45,059 | 1.1 | Calculated |
| 20 | 5 | 50,172 | 1.1 | Calculated |
| 21 | 5 | 54,997 | 1.1 | Calculated |
| 22 | 4 | 57,891 | 1.1 | Calculated |
| 23 | 4 | 61,751 | 1.1 | Calculated |
| 24 | 4 | 67,540 | 1.1 | Calculated |
| 25 | 4 | 73,329 | 1.1 | Calculated |
| 26 | 4 | 79,118 | 1.1 | Calculated |
| 27 | 4 | 84,907 | 1.1 | Calculated |
| 28 | 4 | 89,731 | 1.1 | Calculated |
| 29 | 4 | 95,520 | 1.1 | Calculated |
| 30 | 4 | 102,274 | 1.1 | Calculated |
| 31 | 4 | 107,099 | 1 | Calculated |
| 32 | 4 | 112,888 | 1.1 | Calculated |
| 33 | 4 | 118,677 | 1.1 | Calculated |
| 34 | 4 | 124,466 | 1 | Calculated |
| 35 | 4 | 130,255 | 1 | Calculated |
| 36 | 4 | 156,789 | 1.2 | Calculated |
| 37 | 4 | 162,192 | 1 | Calculated |
| 38 | 4 | 167,884 | 1 | Calculated |
| 39 | 4 | 173,867 | 1 | Calculated |
| 40 | 4 | 179,656 | 1 | Calculated |
| 41 | 4 | 185,252 | 1 | Calculated |
| 42 | 4 | 194,321 | 1 | Calculated |
| 43 | 4 | 200,207 | 1 | Calculated |
| 44 | 4 | 206,479 | 1 | Calculated |
| 45 | 4 | 212,461 | 1 | Calculated |
| 46 | 4 | 236,100 | 1.1 | Calculated |
| 47 | 4 | 241,985 | 1 | Calculated |
| 48 | 4 | 248,353 | 1 | Calculated |
| 49 | 4 | 254,239 | 1 | Calculated |
| 50 | 4 | 275,369 | 1.1 | Calculated |
| 51 | 4 | 282,220 | 1 | Calculated |
| 52 | 4 | 298,526 | 1.1 | Calculated |
| 53 | 4 | 305,347 | 1 | Estimated |
| 54 | 3 | 505,004 | 1.7 | Calculated |
| 55 | 3 | 518,223 | 1 | Calculated |
| 56 | 3 | 532,406 | 1 | Calculated |
| 57 | 3 | 547,458 | 1 | Calculated |
| 58 | 3 | 561,931 | 1 | Calculated |
| 59 | 3 | 575,921 | 1 | Calculated |
| 60 | 3 | 602,165 | 1 | Calculated |
| 61 | 3 | 616,734 | 1 | Calculated |
| 62 | 3 | 632,944 | 1 | Calculated |
| 63 | 3 | 647,803 | 1 | Calculated |
| 64 | 3 | 689,484 | 1.1 | Calculated |
| 65 | 3 | 702,510 | 1 | Calculated |
| 66 | 3 | 716,886 | 1 | Calculated |
| 67 | 3 | 730,394 | 1 | Calculated |
| 68 | 3 | 796,969 | 1.1 | Calculated |
| 69 | 3 | 812,406 | 1 | Calculated |
| 70 | 3 | 842,317 | 1 | Calculated |
| 71 | 3 | 856,790 | 1 | Calculated |
| 72 | 2 | 1,839,975 | 2.1 | Calculated |
| 73 | 2 | 1,875,675 | 1 | Calculated |
| 74 | 2 | 1,914,269 | 1 | Calculated |
| 75 | 2 | 1,949,969 | 1 | Calculated |
| 76 | 2 | 2,197,936 | 1.1 | Calculated |
| 77 | 2 | 2,243,284 | 1 | Calculated |
| 78 | 2 | 2,301,175 | 1 | Calculated |
| 79 | 2 | 2,338,235 | 1 | Calculated |
| 80 | 1 | 9,338,159 | 4 | Calculated |
| 81 | 1 | 9,512,693 | 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.
How does it compare?
Density
11.9 g/cm³21st highest of 99
Melting point
577 K29th lowest of 96
Boiling point
1,746 K34th lowest of 97
Electronegativity
1.6248th lowest of 96
First ionization energy
589 kJ/mol30th 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.
- ThalliumRoyal Society of Chemistry
- ThalliumWikipedia
- Quantum numbers and electron configurationsPurdue
- Hydrogenic wavefunctionsNIST
- Ionization energiesNIST Atomic Spectra Database