Why does francium only have a +1 oxidation state?

Francium has the electron configuration [Rn] 7s1. Losing the single 7s electron reveals the stable [Rn] radon noble-gas core. The second ionization energy is far higher than the first, as it would require breaking into the filled [Rn] core. Like all alkali metals (Li, Na, K, Rb, Cs), francium is strictly +1 in all known chemistry.

Does francium have any stable compounds?

No stable francium compounds exist in macroscopic quantities. Because all francium isotopes are intensely radioactive with short half-lives, no bulk chemistry is possible. Its chemical behavior is predicted from periodic trends (extrapolating from caesium) and from experiments on nanogram or smaller quantities using Fr-221 or Fr-223.

What is francium's electron configuration?

Ground state: [Rn] 7s1, or in full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f0 6s2 6p6 7s1. When ionized to Fr+, the 7s electron is removed, leaving the [Rn] core.

How much francium exists on Earth at any given time?

Estimates put the total amount of naturally occurring francium on Earth at less than 30 grams at any moment. It is produced by the alpha decay of actinium-227 and is always present in trace amounts, but decays away almost immediately. It is the rarest naturally occurring element.

How does francium compare to caesium?

Francium sits directly below caesium in Group 1. By periodic trends, it should have a lower first ionization energy, larger atomic radius, higher reactivity, and a more negative standard reduction potential. However, relativistic effects on the 7s electron slightly increase its ionization energy relative to the simple extrapolated trend, making it slightly less reactive than a naive group trend would predict.

What is the longest-lived francium isotope?

Fr-223 (also written 223Fr) has the longest half-life at approximately 22 minutes. It is the only naturally occurring francium isotope, produced in the radioactive decay chain of U-235 via actinium-227. All other francium isotopes are shorter-lived, most lasting only seconds.

Francium Oxidation States

Reference for francium oxidation states: Fr has exclusively +1 in all known chemistry, with electron configuration, ionization energies, and radioactivity notes.

Atomic # 87 Fr Francium

Atomic Mass

223 u (Fr-223)

Group

1 (IA)

Period

Block

s-block

Electronegativity

0.79 (estimated)

Oxidation States

+1 (only)

Radioactive element. Francium has no stable isotopes. All chemistry is studied on nanogram or sub-nanogram quantities using short-lived Fr-223 (t½ = 22 min). Most listed properties are predicted from periodic trends or measured in ultracold atom traps.

Francium exhibits one known oxidation state: +1. Its ground-state configuration is [Rn] 7s¹ — losing the single 7s valence electron reveals the stable [Rn] noble-gas core. Like all alkali metals, the jump from first to second ionization energy is enormous, making +2 or higher states entirely inaccessible under any ordinary conditions. Because francium is so intensely radioactive, no bulk compounds have ever been synthesized, and its +1 chemistry is known only from periodic extrapolation and nanoscale experiments.

Oxidation State	Stability	Notes
+1	Only known state	The universal state in all known francium chemistry. Fr loses its 7s¹ electron to achieve the [Rn] configuration. Chemistry studied only at nanogram scale or below due to extreme radioactivity.
0	Elemental only	Assigned to pure francium metal by convention. Bulk metallic francium has never been isolated; properties are extrapolated from Group 1 trends (expected soft, silvery-white metal, mp ~8 °C estimated).
+2	Not accessible	Would require removing an electron from the stable [Rn] core. IE₂ is estimated at ~2100 kJ/mol — more than 5× IE₁. No +2 francium compounds are known or predicted under ordinary conditions.

Ionization Energies
IE₁ ≈ 392.8 kJ/mol | IE₂ ≈ 2100 kJ/mol (estimated)
Note: IE₁ of francium is slightly higher than caesium's 375.7 kJ/mol due to relativistic contraction of the 7s orbital — a quantum-relativistic effect that stabilizes s-electrons in very heavy atoms.

Relativistic effect on ionization energy
Simple periodic-table trends predict that ionization energy should keep falling as you go down Group 1 (Li → Na → K → Rb → Cs → Fr). In reality, francium's IE₁ is slightly higher than caesium's because the 7s electron experiences a relativistic mass increase at high nuclear charge (Z = 87), drawing it closer to the nucleus and binding it more tightly. This effect is small but measurable and is the same phenomenon that makes gold yellow and mercury liquid at room temperature.

Ground State Configuration

Core

+10

Core

Core (completes [Rn])

Valence — lost on 1st ionization

Notation

Noble gas: [Rn] 7s¹

Fr⁺ ion: [Rn] (identical to radon noble-gas core)

Ionization Steps

[Rn] 7s¹

Neutral atom — 1 valence electron in 7s

Fr⁺

[Rn]

Remove 7s¹ electron (IE₁ ≈ 392.8 kJ/mol) — stable [Rn] core

Fr²⁺

[Rn]* −e⁻

Break into [Rn] core (IE₂ ≈ 2100 kJ/mol) — not accessible

Relativistic contraction
The 7s orbital contracts slightly due to relativistic effects at Z = 87. This raises IE₁ above the naive group-trend prediction, making Fr marginally harder to ionize than a simple downward extrapolation from caesium would suggest — yet +1 remains the only accessible state.

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Summary

Francium (Fr) is a Group 1 alkali metal with atomic number 87 and the heaviest member of the alkali metal series. It has exactly one known oxidation state — +1 — achieved by losing its single 7s valence electron to attain the stable [Rn] radon noble-gas core. Francium is the most unstable naturally occurring element, with no stable isotopes; its longest-lived isotope, Fr-223, has a half-life of just 22 minutes. This reference covers its electron configuration, ionization energies, predicted properties, and why its chemistry is almost entirely theoretical.

How it works

Click a tab — Oxidation States, Electron Config, or Properties — to explore each area.
The Oxidation States panel explains why +1 is the only accessible state, with a table of known and theoretical states.
The Electron Config panel shows orbital filling and the ionization step to Fr+.
The Properties panel lists atomic and predicted physical data, noting which values are measured vs. estimated.
Click any monospace table cell to copy its value to the clipboard.

Use cases

Students studying Group 1 alkali metal trends and periodic table patterns.
Chemistry teachers explaining why francium behaves like other alkali metals despite being radioactive.
Researchers studying exotic radioactive species or relativistic effects in heavy elements.
Anyone preparing for exams covering Period 7 or Group 1 periodicity.
Science enthusiasts curious about the rarest naturally occurring element on Earth.

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Last updated: 2026-06-19 · Reviewed by Nham Vu