What is the electron configuration of technetium?

Technetium (Tc, Z=43) has the ground-state electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d⁵ 5s², or [Kr] 4d⁵ 5s² in noble-gas shorthand. It has 43 electrons distributed across 9 subshells.

Why does technetium have no stable isotopes?

Technetium has atomic number 43, an odd proton number. Elements with odd Z tend to have fewer stable isotopes because nuclear pairing energy favors even proton counts. More critically, the proton-to-neutron ratio for nuclei near Z=43 falls in a region where no combination yields a stable nucleus — every isotope undergoes beta decay or electron capture. This is a consequence of nuclear structure, not electron configuration.

What is technetium-99m and why is it used in medicine?

⁹⁹ᵐTc is a metastable nuclear isomer of technetium-99. It decays by gamma emission (140 keV) with a 6-hour half-life — long enough for imaging procedures, short enough to minimize patient dose. Hospitals produce it on demand from molybdenum-99 generator systems. It is used in bone scans, cardiac stress tests, thyroid imaging, and more than 40 million procedures per year globally.

How does technetium's configuration compare to molybdenum and ruthenium?

Molybdenum (Z=42) has an anomalous configuration [Kr] 4d⁵ 5s¹ — it sacrifices one 5s electron to achieve a half-filled 4d⁵ shell. Technetium (Z=43) has [Kr] 4d⁵ 5s² — the 5s subshell is filled and the 4d is half-filled by straightforward Aufbau filling. Ruthenium (Z=44) has the anomalous [Kr] 4d⁷ 5s¹, again preferring d-subshell over s. Technetium sits between two anomalous neighbors but follows a regular configuration itself.

Is technetium found naturally?

Technetium does not occur in nature in meaningful quantities. Trace amounts arise from spontaneous fission of uranium-238 in uranium ores, but these are negligible. All practical technetium is synthetic, produced in nuclear reactors as a fission product of uranium-235 or by neutron bombardment of molybdenum-98. It was the first artificially produced element, synthesized by Perrier and Segré in 1937.

What are the valence electrons of technetium?

Technetium's valence electrons are the 4d and 5s electrons in the outermost unfilled shells: 4d⁵ 5s². This gives 7 valence electrons, consistent with its Group 7 position. Technetium can exhibit oxidation states from −1 to +7, with +7 (in pertechnetate, TcO₄⁻) being common and +4 relevant in nuclear medicine compounds.

What does the half-filled 4d⁵ mean for Tc's magnetic properties?

Five unpaired electrons in the 4d subshell (one per orbital, all spin-up per Hund's rule) make technetium paramagnetic. Compounds and complexes of Tc can show a magnetic moment corresponding to 1–5 unpaired electrons depending on the oxidation state and ligand field. The pure metal is weakly paramagnetic.

Technetium Electron Configuration

Name: Technetium Electron Configuration
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Author: Nham Vu

Technetium (Tc, Z=43) ground-state configuration is [Kr] 4d⁵ 5s². Explore its full subshell breakdown, orbital box diagram, quantum numbers, and comparison with neighboring elements Mo and Ru.

Z = 43 Tc Technetium

Technetium — Electron Configuration

Atomic number 43 · Transition metal · Period 5, Group 7 · d-block · No stable isotopes

[Kr] 4d⁵ 5s² 43 electrons 7 valence e⁻ Artificial element Paramagnetic

Full Subshell Breakdown — 43 Electrons

Subshell	Type	Electrons	Capacity	Notation	Core
1s	s orbital, shell n=1	2	2	1s²	[Kr] core
2s	s orbital, shell n=2	2	2	2s²	[Kr] core
2p	p orbitals (3), shell n=2	6	6	2p⁶	[Kr] core
3s	s orbital, shell n=3	2	2	3s²	[Kr] core
3p	p orbitals (3), shell n=3	6	6	3p⁶	[Kr] core
3d	d orbitals (5), shell n=3	10	10	3d¹⁰	[Kr] core
4s	s orbital, shell n=4	2	2	4s²	[Kr] core
4p	p orbitals (3), shell n=4	6	6	4p⁶	[Kr] core
4d	d orbitals (5), shell n=4	5	10	4d⁵	valence
5s	s orbital, shell n=5	2	2	5s²	valence
Total		43

Full Configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d⁵ 5s²

All 9 occupied subshells listed explicitly.

Noble-Gas Shorthand

[Kr] 4d⁵ 5s²

[Kr] = 1s²…4p⁶ (krypton's 36-electron core).

Valence Shell

4d⁵ 5s²

7 valence electrons; Group 7 identity. Highest common oxidation state: +7.

Shell Fill Summary

Shell 1 (n=1) — 1s² 2 / 2 electrons (100%)

Shell 2 (n=2) — 2s² 2p⁶ 8 / 8 electrons (100%)

Shell 3 (n=3) — 3s² 3p⁶ 3d¹⁰ 18 / 18 electrons (100%)

Shell 4 (n=4) — 4s² 4p⁶ 4d⁵ 13 / 32 electrons (41%)

Shell 5 (n=5) — 5s² 2 / 50 electrons (4%)

Shell 4 can hold up to 32 electrons (4s + 4p + 4d + 4f). Technetium fills 13 of those 32 slots — the 4f subshell is empty. Shell 5 has only the 5s² pair; the 5p, 5d, 5f orbitals are empty.

Three Rules That Determine the Configuration

Aufbau Principle

Electrons fill subshells from lowest to highest energy. For Tc, the order reaches 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d, placing the 4d electrons after 5s rather than before it.

Hund's Rule

Within the five 4d orbitals, one electron enters each orbital before any pairing occurs. All five 4d electrons have parallel spin (+½). This half-filled 4d⁵ arrangement is particularly stable.

Pauli Exclusion

No two electrons share the same four quantum numbers (n, l, m_l, m_s). Each orbital holds at most two electrons with opposite spins. The 5s² pair satisfies this with m_s = +½ and −½.

Quantum Number Definitions

n — Principal quantum number. Determines the shell (energy level). Values 1–5 for Tc.

l — Azimuthal quantum number. Determines subshell shape: 0 = s, 1 = p, 2 = d, 3 = f.

m_l — Magnetic quantum number. Distinguishes orbitals within a subshell. Ranges from −l to +l.

m_s — Spin quantum number. Either +½ (spin up ↑) or −½ (spin down ↓) per Pauli exclusion.

Valence Electrons (4d and 5s) — Quantum Numbers

e⁻ #	n	l	m_l	m_s	Subshell	Note
1	1	0	0	+½	1s
2	1	0	0	−½	1s
3	2	0	0	+½	2s
4	2	0	0	−½	2s
5	2	1	-1	+½	2p
6	2	1	-1	−½	2p
7	2	1	0	+½	2p
8	2	1	0	−½	2p
9	2	1	1	+½	2p
10	2	1	1	−½	2p
11	3	0	0	+½	3s
12	3	0	0	−½	3s
13	3	1	-1	+½	3p
14	3	1	-1	−½	3p
15	3	1	0	+½	3p
16	3	1	0	−½	3p
17	3	1	1	+½	3p
18	3	1	1	−½	3p	[Kr] core ends here at e⁻ 36
19	3	2	-2	+½	3d
20	3	2	-2	−½	3d
21	3	2	-1	+½	3d
22	3	2	-1	−½	3d
23	3	2	0	+½	3d
24	3	2	0	−½	3d
25	3	2	1	+½	3d
26	3	2	1	−½	3d
27	3	2	2	+½	3d
28	3	2	2	−½	3d
29	4	0	0	+½	4s
30	4	0	0	−½	4s
31	4	1	-1	+½	4p
32	4	1	-1	−½	4p
33	4	1	0	+½	4p
34	4	1	0	−½	4p
35	4	1	1	+½	4p
36	4	1	1	−½	4p	[Kr] core ends at e⁻ 36
37	4	2	-2	+½	4d	Hund: unpaired spin-up
38	4	2	-1	+½	4d	Hund: unpaired spin-up
39	4	2	0	+½	4d	Hund: unpaired spin-up
40	4	2	1	+½	4d	Hund: unpaired spin-up
41	4	2	2	+½	4d	Hund: unpaired spin-up
42	5	0	0	+½	5s	Paired valence s
43	5	0	0	−½	5s	Paired valence s

Highlighted rows (37–43) are valence electrons outside the [Kr] core. n = principal; l: 0=s, 1=p, 2=d; m_l = magnetic; m_s = spin.

Mo, Tc, Ru — Electron Configurations Compared

Element	Z	Configuration	4d electrons	Anomalous?
Molybdenum (Mo)	42	[Kr] 4d⁵ 5s¹	5 (half-filled)	Yes — 5s¹ not 5s²
Technetium (Tc)	43	[Kr] 4d⁵ 5s²	5 (half-filled)	No — regular Aufbau
Ruthenium (Ru)	44	[Kr] 4d⁷ 5s¹	7	Yes — 5s¹ not 5s²

Why Mo and Ru Are Anomalous but Tc Is Not

Molybdenum (Z=42) has only one electron to go into the 4d subshell after filling [Kr] 4s² 4p⁶ — if it followed strict Aufbau, it would be [Kr] 4d⁴ 5s². Instead, one 5s electron promotes to 4d, giving [Kr] 4d⁵ 5s¹. This achieves a half-filled 4d⁵ configuration with additional exchange-energy stability, outweighing the cost of a half-empty 5s.

Technetium (Z=43) adds one more electron to Mo's configuration. With [Kr] 4d⁵ 5s² it already has the half-filled 4d⁵ that Mo sought — no promotion is needed. The 5s subshell fills naturally. Tc is the only Group 7 metal that follows regular Aufbau in the period.

Ruthenium (Z=44) has [Kr] 4d⁷ 5s¹ — again anomalous, favoring a d-heavy configuration. This pattern of anomalous 5s electron counts recurs across the 4d transition series (Nb, Mo, Ru, Rh, Pd all deviate from expected 5s²).

Periodic Table Position

Period 5

Row 5 of the periodic table; valence electrons enter n=4 d and n=5 s subshells

Group 7

Same column as Mn (Z=25) and Re (Z=75); all have 7 valence electrons

d-block

Transition metal; distinguishing electron enters the 4d subshell

+7 max

Maximum oxidation state matches group number; pertechnetate TcO₄⁻ is common

Summary

Technetium (Tc, Z=43) ground-state configuration is [Kr] 4d⁵ 5s². Explore its full subshell breakdown, orbital box diagram, quantum numbers, and comparison with neighboring elements Mo and Ru.

How it works

The Aufbau principle fills subshells in increasing energy order: 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d for technetium.
After filling the [Kr] core (36 electrons), the next 2 electrons enter 5s, then 5 electrons fill the 4d subshell.
Hund's rule places one electron in each of the five 4d orbitals before any pairing — all five have parallel spins (+½).
The Pauli exclusion principle ensures no two electrons in any orbital share the same set of four quantum numbers.
The result is [Kr] 4d⁵ 5s²: a half-filled d subshell with notable stability, similar to the pattern in chromium and molybdenum.
Use the tabs below to explore the configuration table, orbital box diagram, quantum numbers, and element comparisons.

Use cases

Quick chemistry homework reference for Period 5 transition metal configurations.
Understand why the half-filled 4d⁵ arrangement is significant in transition metal chemistry.
Study the relationship between electron configuration and technetium's radioactive properties.
Learn why ⁹⁹ᵐTc is the radioisotope of choice in nuclear medicine imaging.
Compare electron configurations across Mo (Z=42), Tc (Z=43), and Ru (Z=44).
Teaching aid for Aufbau principle, Hund's rule, and Pauli exclusion applied to d-block elements.

Frequently Asked Questions

Last updated: 2026-07-23 · Reviewed by Nham Vu