Thallium (Tl)
Element 81 · post-transition metal · group 13, period 6
Thallium is element 81, a post-transition metal in group 13 and period 6. Its electron configuration is [Xe] 4f14 5d10 6s2 6p1. That leaves 3 valence electrons in the outer shell, with the shells filling 2, 8, 18, 32, 18, 3. Losing its 3 valence electrons is the shorter route to a full shell, so it carries a charge of +1, +3.
- 81 protons
- 3 valence e−
- +1, +3
- p-block
Properties
| Atomic number | 81 |
|---|---|
| Symbol | Tl |
| Standard atomic weight | 204.38 u |
| Group | 13 |
| Period | 6 |
| Block | p-block |
| Category | post-transition metal |
| Electron configuration | 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p1 |
| Noble-gas notation | [Xe] 4f14 5d10 6s2 6p1 |
| Electrons per shell | 2, 8, 18, 32, 18, 3 |
| Valence electrons | 3 |
| Common ionic charge | +1, +3 |
| Electronegativity (Pauling) | 1.62 |
| Protons / electrons | 81 / 81 |
Electron configuration of thallium
Orbitals fill from the lowest energy upward. Writing that out for thallium gives 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p1, which is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p1 in plain text.
Everything up to Xe is the same as in that noble gas, so it is usually abbreviated to [Xe] 4f14 5d10 6s2 6p1. The part outside the bracket is what does the chemistry.
| Subshell | Electrons | Capacity |
|---|---|---|
| 1s | 2 | 2 |
| 2s | 2 | 2 |
| 2p | 6 | 6 |
| 3s | 2 | 2 |
| 3p | 6 | 6 |
| 3d | 10 | 10 |
| 4s | 2 | 2 |
| 4p | 6 | 6 |
| 4d | 10 | 10 |
| 4f | 14 | 14 |
| 5s | 2 | 2 |
| 5p | 6 | 6 |
| 5d | 10 | 10 |
| 6s | 2 | 2 |
| 6p | 1 | 6 |
Thallium ion charge
Thallium commonly forms ions with a charge of +1, +3.
It has 3 valence electrons, so it is 5 short of a full octet and 3 above an empty outer shell. An atom takes whichever route is shorter, which is why the charge comes out as it does.
Common questions
How many valence electrons does thallium have?
Thallium has 3 valence electrons. For a main-group element the count is the group number minus 10, and thallium is in group 13.
What is the electron configuration of thallium?
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p1, or [Xe] 4f14 5d10 6s2 6p1 written from the previous noble gas. Orbitals fill lowest energy first.
What is the charge of thallium?
Thallium commonly forms ions with a charge of +1, +3. With 3 valence electrons it is 5 short of an octet and 3 above an empty shell, so it takes whichever route is shorter.
How many protons, electrons and neutrons does thallium have?
A neutral thallium atom has 81 protons and 81 electrons. Its standard atomic weight is 204.38, so the most common isotope has about 123 neutrons.
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