Thallium (Tl)
Thallium is element 81, a post-transition metal in group 13, period 6. A neutral atom has 81 protons, 123 neutrons and 81 electrons, 3 of them valence electrons.
Post-transition metal- Atomic number
- 81
- Atomic mass
- 204.38 u
- Valence electrons
- 3
- Protons · neutrons
- 81 · 123
- Electrons per shell
- 2, 8, 18, 32, 18, 3
- At room temperature
- solid
Bohr model of thallium
The Bohr model draws thallium as a nucleus of 81 protons and 123 neutrons, with its 81 electrons on 6 circular shells: 2 in shell 1, 8 in shell 2, 18 in shell 3, 32 in shell 4, 18 in shell 5, 3 in shell 6. Press Build the atom to watch the shells fill from the inside out. The outer ring (the P shell) is dashed: the electrons there are the ones that take part in chemistry.
Real electrons don't travel in neat circles. They live in fuzzy orbitals, shown below, but the Bohr picture is the quickest way to count shells and valence electrons.
Electron configuration of thallium
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p¹
Short form (noble gas notation): [Xe] 6s² 4f¹⁴ 5d¹⁰ 6p¹. The [Xe] stands for the electrons of xenon, the noble gas before it.
How many valence electrons does thallium have?
Thallium has 3 valence electrons.
Thallium is in group 13, and for groups 13–18 you subtract 10 from the group number: 13 − 10 = 3. It tends to lose them and form a +1 ion.
Protons, neutrons and electrons
| Protons | 81 | equal to the atomic number |
|---|---|---|
| Electrons | 81 | equal to the protons in a neutral atom |
| Neutrons | 123 | mass number 204 − 81 protons |
Atoms of the same element can have different numbers of neutrons (isotopes). The count above uses the rounded atomic mass, the way it's taught in class.
Thallium properties
| Symbol | Tl |
|---|---|
| Category | Post-transition metal |
| Group · period · block | 13 · 6 · p |
| Atomic mass | 204.38 u |
| State at 25 °C | solid |
| Melting point | 577 K (304 °C, 579 °F) |
| Boiling point | 1,746 K (1,470 °C, 2,680 °F) |
| Density | 11.8 g/cm³ |
| Electronegativity (Pauling) | 1.62 |
| Atomic radius | 196 pm |
| First ionization energy | 6.108 eV |
| Electron affinity | 0.2 eV |
| Oxidation states | +3, +1 |
| Common ion charge | +1, +3 |
| Discovered | 1861 by William Crookes |
Where thallium sits on the periodic table
Group 13: Boron, Aluminum, Gallium, Indium, Nihonium.
Questions about thallium
How many valence electrons does thallium have?
Thallium has 3 valence electrons.
How many protons, neutrons and electrons does thallium have?
A neutral thallium atom has 81 protons and 81 electrons. Its most common form has about 123 neutrons (mass number 204 − 81 protons).
What is the electron configuration of thallium?
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p¹, written in short form as [Xe] 6s² 4f¹⁴ 5d¹⁰ 6p¹.
What does the Bohr model of thallium look like?
A nucleus of 81 protons and 123 neutrons, circled by 6 electron shells holding 2, 8, 18, 32, 18, 3 electrons.
What is the atomic number of thallium?
81. The atomic number is the number of protons in the nucleus, and it is what makes an atom thallium.