Technetium at a glance
- Atomic number
- 43
- Symbol
- Tc
- Atomic mass
- 98 u
- Category
- Transition metal
- Group / period
- 7 / 5
- Electron configuration
- [Kr] 4d 5 5s 2
- State at 25 °C
- Solid
- Melting point
- 2157 °C
- Boiling point
- 4265 °C
- Density
- 11.5 g/cm³
- Electronegativity
- 1.9
- 1st ionization energy
- 702 kJ/mol
- Oxidation states
- +7, +4
- Discovered
- 1937
Atomic structure
A neutral technetium atom has 43 protons and 43 electrons, and its most common isotope carries about 55 neutrons. One atom weighs about 1.63 × 10⁻²⁵ kg, so a mole of technetium (6.022 × 10²³ atoms) weighs 98 g. That nucleus holds 1.28 neutrons for every proton. The electrons fill 5 shells (K 2, L 8, M 18, N 13, O 2), which gives the configuration [Kr] 4d 5 5s 2. As a d-block element, both its outer s electrons and its inner d electrons can take part in bonding, which is why it shows more than one oxidation state. All of its isotopes are radioactive, so any technetium on Earth is either freshly made or part of a decay chain.
Physical properties
At room temperature (25 °C) technetium is a solid. It melts at 2157 °C and boils at 4265 °C, so it stays liquid across a 2108-degree range. It stays solid in molten iron (1538 °C) but would melt in a welding arc. That is the 11th highest melting point of any element. Its density is 11.5 g/cm³, 11.5 times as dense as water. Its atoms have a radius of about 135 pm (Molybdenum next door is 145 pm). In the solid, its atoms pack into a hexagonal close-packed lattice.
Chemical behaviour
Its electronegativity is 1.9 on the Pauling scale. Pulling off its outermost electron takes 702 kJ/mol, less than Manganese above it (717 kJ/mol). A bond between technetium and chlorine would be polar covalent, with an electronegativity difference of 1.3. It releases 53 kJ/mol when it gains an extra electron. Its common oxidation states are +7, +4, so in compounds you will usually meet it as Tc⁷⁺ and Tc⁴⁺. Group 7 also holds Manganese, Rhenium and Bohrium, which share its outer-shell pattern. Handle with care: as a pure element it is radioactive.
Where it comes from
It is vanishingly rare in nature, existing only briefly as part of radioactive decay chains. It was first identified in 1937, making it the 89th element to be discovered (counting the 11 known since antiquity together).