Chemical element · Atomic number 90
Thorium
Thorium in the periodic table: atomic number 90, electron configuration, atomic mass, physical data, oxidation states, media credit and visible sources.
Actinide
solid
232.038 u
Documented element sampleAn ampoule holding thorium metal; the surface of the sample has already tarnished.
Image credit: W. Oelen
Auto-oriented, limited to 1600 × 1200 pixels and re-encoded as WebP; the subject was not altered.
Atomic classification
Shell occupancy
Thorium in the Bohr shell model
This shows the electron distribution of the neutral atom in a simplified shell model.
- K · n=1
- 2 electrons
- L · n=2
- 8 electrons
- M · n=3
- 18 electrons
- N · n=4
- 32 electrons
- O · n=5
- 18 electrons
- P · n=6
- 10 electrons
- Q · n=7
- 2 electrons
- Electron configuration
- [Rn]7s2 6d2
- Electrons per shell
- 2 · 8 · 18 · 32 · 18 · 10 · 2
- Group
- not reported
- Period
- 7
- Block
- F
- Element category
- Actinide
Physical and chemical properties
- Atomic mass
- 232.038 u
- Standard state
- solid
- Density
- 11.72 g/cm³
- Melting point
- 2,023 K
- Boiling point
- 5,061 K
- Electronegativity
- 1.3 (Pauling)
- First ionisation energy
- 6.08 eV
- Oxidation states
- +4
- Discovery
- 1828
Safety and periodic classification
Safety
Safe handling cannot be inferred from Thorium's position in the periodic table alone. Laboratory, classroom and disposal decisions must follow the documentation for the exact material and its safety data sheet.
Position and comparison
Thorium is in period 7 and the F block. Its direct neighbours by atomic number are Actinium and Protactinium. The recorded Pauling electronegativity is 1.3. Periodic trends are compared only through the separately sourced neighbouring values.
Sources and scope
PubChem attributes element data to sources including IUPAC, NIST and IAEA. Quanta stores the referenced snapshot locally and leaves unknown values unavailable.
Where Thorium sits among the 118 elements
The table compares the reported values for Thorium with every other element that carries the same value in the snapshot. A rank is not a judgement, it is orientation: it says how unusual this value is across the periodic table.
| Value | Thorium | Position in the data set |
|---|---|---|
| Melting point | 2,023 K | Rank 18 of 103 elements with a reported value. Highest: Carbon. |
| Boiling point | 5,061 K | Rank 5 of 93 elements with a reported value. Highest: Rhenium. |
| Density | 11.72 g/cm³ | Rank 21 of 96 elements with a reported value. Highest: Osmium. |
| Electronegativity (Pauling) | 1.3 | Rank 62 of 95 elements with a reported value. Highest: Fluorine. |
| Ionisation energy | 6.08 eV | Rank 74 of 102 elements with a reported value. Highest: Helium. |
All values and ranks come from the PubChem Periodic Table snapshot, retrieved on 2026-07-15. Elements without a reported value are excluded, so the population differs per row.
The neighbours of Thorium in the periodic table
- Place in the table
- No group number, period 7, F block. Classified as actinide, solid at standard conditions.
- Direct neighbours
- Atomic number 90. Preceded by Actinium, followed by Protactinium.
- Discovery
- Recorded in the snapshot as: 1828. Who, where and under what circumstances is not part of that record, so this page carries the sourced year and no discovery narrative.
- Oxidation states
- +4. They follow from the electron configuration [Rn]7s2 6d2 and indicate which compounds to expect.
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