Thorium (Th)
Thorium is element 90, an actinide in the actinide series (period 7). A neutral atom has 90 protons, 142 neutrons and 90 electrons, 2 of them valence electrons.
Actinide- Atomic number
- 90
- Atomic mass
- 232.04 u
- Valence electrons
- 2*
- Protons · neutrons
- 90 · 142
- Electrons per shell
- 2, 8, 18, 32, 18, 10, 2
- At room temperature
- solid
Bohr model of thorium
The Bohr model draws thorium as a nucleus of 90 protons and 142 neutrons, with its 90 electrons on 7 circular shells: 2 in shell 1, 8 in shell 2, 18 in shell 3, 32 in shell 4, 18 in shell 5, 10 in shell 6, 2 in shell 7. Press Build the atom to watch the shells fill from the inside out. The outer ring (the Q 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 thorium
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 6d²
Short form (noble gas notation): [Rn] 7s² 6d². The [Rn] stands for the electrons of radon, the noble gas before it.
Thorium breaks the usual filling order. The Aufbau rule predicts 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f², but experiments show the configuration above. The energy levels of the subshells are so close here that a different arrangement wins.
How many valence electrons does thorium have?
Thorium has 2 valence electrons.
Thorium is a actinide, so its valence electrons are less clear-cut than for main-group elements. It has 2 electrons in its outermost shell (shell 7), and electrons in its inner f and d subshell can also take part in bonding.
Protons, neutrons and electrons
| Protons | 90 | equal to the atomic number |
|---|---|---|
| Electrons | 90 | equal to the protons in a neutral atom |
| Neutrons | 142 | mass number 232 − 90 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.
Thorium properties
| Symbol | Th |
|---|---|
| Category | Actinide |
| Group · period · block | — · 7 · f |
| Atomic mass | 232.04 u |
| State at 25 °C | solid |
| Melting point | 2,023 K (1,750 °C, 3,180 °F) |
| Boiling point | 5,061 K (4,790 °C, 8,650 °F) |
| Density | 11.72 g/cm³ |
| Electronegativity (Pauling) | 1.3 |
| Atomic radius | 237 pm |
| First ionization energy | 6.08 eV |
| Electron affinity | — |
| Oxidation states | +4 |
| Common ion charge | +4 |
| Discovered | 1828 by Jöns Jakob Berzelius |
Where thorium sits on the periodic table
Actinides: Actinium, Protactinium, Uranium, Neptunium, Plutonium, Americium, Curium, Berkelium.
Questions about thorium
How many valence electrons does thorium have?
Thorium has 2 valence electrons in its outer shell; inner d or f electrons can also bond.
How many protons, neutrons and electrons does thorium have?
A neutral thorium atom has 90 protons and 90 electrons. Its most common form has about 142 neutrons (mass number 232 − 90 protons).
What is the electron configuration of thorium?
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 6d², written in short form as [Rn] 7s² 6d².
What does the Bohr model of thorium look like?
A nucleus of 90 protons and 142 neutrons, circled by 7 electron shells holding 2, 8, 18, 32, 18, 10, 2 electrons.
What is the atomic number of thorium?
90. The atomic number is the number of protons in the nucleus, and it is what makes an atom thorium.