Rhodium Electron Configuration and Oxidation States

Rhodium's ground-state electron configuration is [Kr] 4d8 5s1, an arrangement that breaks the naive Aufbau filling order a simple periodic-table position would predict. Rhodium (Rh, Z=45) is one of the six platinum group metals (PGMs), a family whose catalytic value depends less on any single dominant oxidation state than on the coordination chemistry each metal's configuration makes possible. The sections below give rhodium's identity data, its two commercially significant oxidation states, and how each one shows up in a named rhodium compound, checked against Wikipedia, the CRC Handbook of Chemistry and Physics and PubChem.

What Is Rhodium's Electron Configuration?#

Rhodium's condensed ground-state electron configuration is [Kr] 4d8 5s1, a krypton core plus eight 4d electrons and one 5s electron. That places the element in period 5, group 9, in the d-block, with 9 valence electrons across the 4d and 5s levels, per the periodic-table identity data compiled from Wikipedia's element infoboxes, which in turn cite the CRC Handbook of Chemistry and Physics and Greenwood and Earnshaw's Chemistry of the Elements. Rhodium's elemental CAS Registry Number is 7440-16-6, the identifier for the metal itself, and it is not interchangeable with a compound-specific number such as 10049-07-7, assigned to anhydrous rhodium(III) chloride. The table below sets out the full identity record for rhodium (Rh), element 45.

Is Rhodium's Configuration an Aufbau Exception?#

Yes, rhodium's configuration is an Aufbau exception. The naive filling order predicts [Kr] 4d7 5s2, but rhodium's actual ground state is [Kr] 4d8 5s1, promoting one 5s electron into the 4d subshell. The 4d and 5s orbitals sit close enough in energy across the second transition-metal row that the promotion lowers the atom's total energy, and rhodium is one of six elements in this identity dataset where the naive order fails, alongside ruthenium ([Kr] 4d7 5s1), palladium ([Kr] 4d10), silver, platinum and gold. Only two elements in the same comparison, osmium and iridium, follow the naive filling order exactly and keep their expected 6s2 pair intact; rhodium's own group-9 neighbor, iridium ([Xe] 4f14 5d7 6s2), is one of those two, a contrast set out fully in the group-neighbor comparison further down this page.

Rhodium Oxidation States#

Rhodium's two commercially significant oxidation states are +1 and +3, out of a full observed range that spans -3 to +7 for the element, according to Wikipedia's rhodium element data. Rhodium(I) is square planar with a d8 electron count, the geometry behind Wilkinson's catalyst, while rhodium(III) is octahedral with a d6 count, the geometry of rhodium(III) chloride and its trihydrate. The table below lists an example compound and coordination geometry for each of rhodium's principal oxidation states.

Rhodium hexafluoride, the highest oxidation state isolated as a solid, begins decomposing at room temperature and attacks glass, making it far less stable than the +6 fluorides of its heavier neighbors osmium and iridium. A +7 state is recorded only in gas-phase rhodium oxide cation chemistry, never in an isolable compound, and rhodium(II) exists only as the dimeric paddlewheel structure shown above, never as a simple mononuclear ion. Because rhodium's +1 and +3 chemistry underpins the autocatalysts that consumed 84.6% of rhodium supply in 2025 (Johnson Matthey, May 2026), this same oxidation-state chemistry also drives the industrial demand behind rhodium price today.

Electron Configuration and Rhodium Chemistry#

Rhodium's [Kr] 4d8 5s1 configuration enables two catalytically dominant geometries: square-planar rhodium(I), a d8 center, and octahedral rhodium(III), a d6 center. The d8 rhodium(I) state is demonstrated most famously by Wilkinson's catalyst, RhCl(PPh3)3, a coordinatively unsaturated 16-electron complex that hydrogenates simple alkenes in solution at room temperature and one atmosphere of hydrogen gas. The d6 rhodium(III) state appears in rhodium(III) chloride trihydrate, RhCl3·3H2O, the water-soluble precursor from which Wilkinson's catalyst and most other rhodium catalysts and plating baths are made; the anhydrous form of the same chloride, by contrast, is essentially inert and insoluble in both water and acids. This +1/+3 pair, not the wider -3 to +7 range, is what carries essentially all of rhodium's industrial catalysis, per the coordination chemistry compiled from Wikipedia's element and compound entries.

Rhodium vs Its Group Neighbours#

Rhodium's closest group neighbor is iridium, element 77, directly below it in group 9, and the two metals diverge sharply in how their electron configurations fill. Rhodium is Aufbau-anomalous, promoting a 5s electron into the 4d subshell, while iridium's electron configuration follows the naive filling order exactly and keeps its full 6s2 pair intact. The two metals also diverge in oxidation-state profile: rhodium's dominant states are +1 and +3, iridium adds a dominant +4 alongside the same +1 and +3, and iridium alone among all elements reaches +9, in the gas-phase [IrO4]+ cation, the highest oxidation state recorded for any element. The table below sets the two group-9 metals side by side.

Electron Configuration of the Other PGMs#

The other five platinum group metals each have their own ground-state configuration and dominant oxidation states, summarized below rather than restated in prose.

Related Rhodium Attributes#

Rhodium's other core attributes each have their own dedicated page rather than being restated here.

• Rhodium properties: physical and chemical properties
• Rhodium density
• Rhodium melting point and boiling point
• Rhodium uses in industry and automotive catalysis
• Rhodium occurrence, ores and minerals
• Rhodium discovery and history
• Rhodium isotopes and radioactivity
• Rhodium compounds
• Rhodium toxicity and health effects
• Rhodium magnetic properties

Frequently Asked Questions#

What is the 2, 8, 8, 18 rule in chemistry? The 2, 8, 8, 18 pattern is the Bohr-model shell capacity for the first four periods, since 4s fills before 3d. The same effect is why rhodium's 4d and 5s orbitals fill out of order, addressed above.

What electron configuration is 1s2 2s2 2p6 3s2 3p6 4s2 3d10? That electron configuration belongs to zinc (Zn, Z=30), not rhodium. The two elements share no chemical relationship beyond both appearing on the periodic table.

Is rhodium the rarest element? Rhodium's crustal abundance and market scarcity are covered on this site's rhodium (Rh), element 45 overview, not on this configuration page.

Is rhodium toxic or radioactive? Rhodium's health effects and radioactivity are covered on their own pages: rhodium toxicity and rhodium isotopes. This page covers only electron configuration and oxidation states.

What dissolves rhodium? Rhodium resists nitric acid and dissolves only slightly in aqua regia, protected by a passivating rhodium(III) oxide film, per Wikipedia's rhodium entry. Refiners instead use molten sodium bisulfate, hot sulfuric acid with an oxidant, or high-temperature chlorination.

What metal is 30 times rarer than gold? That comparison belongs to rhodium's pricing and scarcity data, not its atomic structure. See rhodium price today for current context.

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