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Tc

Technetium

Technetium (Tc) is a transition metal with atomic number 43. It has no stable isotope and no characteristic isotopic composition on Earth, so it has no standard atomic weight. CIAAW lists technetium-97 and technetium-98 as its longest-lived isotopes, and the mass number used here is 98. Its electron configuration is [Kr] 4d5 5s2.

Technetium key data

Symbol Tc
Atomic number 43
Mass number (one of its longest-lived isotopes) 98
Protons 43
Neutrons (technetium-98) 55
Electrons (neutral atom) 43
Category Transition metal
Period 5
Group 7
Block d-block
Noble gas notation [Kr] 4d5 5s2
Full electron configuration 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d5 5s2
Electrons per shell 2, 8, 18, 13, 2

Mass number: IUPAC CIAAW radioactive elements, from NUBASE2020.

The values are facts and free to use; this page’s selection and presentation are © 2026 ScienceQuest.

Measured properties of technetium

From PubChem, the US National Institutes of Health. Public domain. The first ionisation energy is the value for technetium from the NIST Atomic Spectra Database, in place of PubChem’s 7.28 eV.

Melting point 2430 K (2156.9 °C), 11th of 103 by melting point
Boiling point 4538 K (4264.9 °C), 11th of 93 by boiling point
Density 11 g/cm³, 23rd of 96 by density
State at room temperature Solid
Electronegativity 1.9 (Pauling), joint 36th of 95 by electronegativity
Van der Waals radius 209 pm, joint 48th of 99 by atomic radius
First ionisation energy 7.1194 eV (686.92 kJ/mol), 50th of 102 by first ionisation energy
Electron affinity 0.55 eV (53.1 kJ/mol), 36th of 57 by electron affinity
Common oxidation states +7, +6, +4
Discovered 1937

Each rank counts down from the highest value, among the elements PubChem’s table gives one for, and links to that property ranked across the whole table. All of them are collected under periodic trends.

Technetium electron configuration

The full electron configuration of technetium is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d5 5s2. With the noble-gas core collapsed it is [Kr] 4d5 5s2, and the outermost subshell is 5s2.

Bohr model of technetium (Tc): a nucleus and five shells holding 2, 8, 18, 13 and 2 electrons from the inside out.
Bohr model of technetium (Tc). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.
Orbital box diagram of technetium (Tc), [Kr] 4d5 5s2, with the krypton core drawn as one bar: one box per orbital, filled by Hund’s rule, showing five unpaired electrons, in 4d.
Orbital diagram of technetium (Tc). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Why group and block follow from this is set out in how the periodic table maps electron configuration, and the electron configuration of every element is listed on one page.

Mass of one technetium atom

A molar mass is grams per mole, so dividing by the Avogadro constant gives the mass of a single atom, or for an element with more than one natural isotope the average mass of its atoms, and the reciprocal gives how many atoms sit in a gram.

Mass of one atom

  1. one atom = 98 / (6.02214076 × 10²³)
  2. = 1.6273 × 10⁻²² g

Atoms in one gram

  1. atoms per gram = 6.02214076 × 10²³ / 98
  2. = 6.1450 × 10²¹ atoms

Technetium has no standard atomic weight, because it has neither a stable isotope nor a characteristic isotopic composition on Earth. The figure used above is the mass number of technetium-98, so treat these as nominal.

Technetium on the periodic table

Technetium is in period 5, group 7, in the d-block.

The other elements of group 7 are manganese, rhenium and bohrium.

The rest of period 5

Work with technetium