Rhodium (Rh)
“The rarest of the precious metals — a pinch rides in every car’s exhaust pipe, and in bumper years it has fetched more than ten times the price of gold.”
How its electrons are arranged
Computed from the aufbau principle, with known real-world exceptions applied automatically.
The notes
Characteristics
- Electron configuration [Kr] 4d⁸ 5s¹ — a genuine Period 5 aufbau exception (plain filling would give [Kr] 4d⁷ 5s²); a Group 9 transition metal, sitting below cobalt and above iridium.
- One of the rarest metals in the crust — roughly one part per billion, several-fold scarcer than gold — and in recent years repeatedly the most expensive metal on Earth.
- Extraordinarily hard and high-melting (1,964 °C), and among the most reflective of metals — a rhodium-plated surface throws back light like a mirror and never tarnishes.
- A catalytic superstar: rhodium alone splits the stubborn NOₓ molecules of exhaust back into harmless nitrogen and oxygen — a job no cheaper metal performs as well.
A brilliant silvery-white metal of startling reflectivity and hardness, and by a wide margin the least abundant precious metal mined today. Consumers never see it in bulk — only as a whisper-thin, mirror-bright plating that refuses to tarnish.
Where you meet it
✦ Uses
- Three-way catalytic converters on modern petrol vehicles — rhodium is the reduction specialist that tears apart NOₓ, while platinum or palladium handles CO and unburnt fuel.
- Rhodium plating on white-gold and silver rings — that dazzling, mirror-like showroom finish is a micron of rhodium, renewed by jewellers every few years.
- Platinum-rhodium gauze catalysts in nitric-acid plants, where ammonia is burned into nitrogen oxides on the road to fertiliser-grade acid.
- Searchlight and headlight reflectors, and premium mirrors, bank on rhodium’s waste-no-light reflectivity even in corrosive city air.
◍ Everyday examples
- Any white-gold wedding ring bought from a Bangladeshi jewellery shop almost certainly wears a rhodium coat — its brightness is literally rhodium’s.
- Every Euro-3-or-newer car, bus or truck squeezing through Dhaka traffic hides a few grams of rhodium in its catalytic converter.
History & name
In 1803 William Hyde Wollaston, dissecting crude South American platinum in his London laboratory, found that once platinum and his brand-new palladium were removed, the leftover solution deposited striking rose-red crystals of a sodium double salt. Warming these in hydrogen gas left a grey-white metal of unmatched stubbornness. He crowned it rhodium, from the Greek rhodon, “rose” — a name earned by the colour of the salts, for the metal itself is plain silvery-white. It was the second new element within twelve months for the reticent Wollaston, who kept his clever platinum-refining process a trade secret for years and grew rich on it.