Palladium – facts and properties
- Symbol
- Pd
- Atomic number
- 46
- Atomic mass
- 106.42 u
- Period
- 5
- Group
- 10
- Category
- transition metal
- Electron configuration
- [Kr] 4d¹⁰
- Electronegativity
- 2.2
- Atomic radius
- 169 pm
- Valence electrons
- 10
- State at standard conditions
- solid
- Melting point
- 1554.9 °C
- Boiling point
- 2963 °C
- Density
- 12 g/cm³
- Discovered
- 1803
Palladium is a silvery-white, shiny precious metal with atomic number 46, one of the platinum metals. It is named after the asteroid Pallas, found shortly before the element was discovered. Palladium is rare and expensive, at times worth more than gold. It cleans car exhaust as a catalyst, can soak up large amounts of hydrogen, and drives chemical reactions that earned a Nobel Prize in 2010.
Position in the periodic table
Occurrence and extraction
Palladium is rare in the Earth's crust. It occurs together with platinum and the other platinum metals, and in traces in nickel-copper ores. Most of it comes from Russia, where it is a by-product of nickel mining, and from South Africa. It is recovered from the anode sludge of nickel and copper refining and from platinum ores. Today a large share also comes from recycling used car catalytic converters.
The element was discovered in 1803 by the English chemist William Hyde Wollaston. He dissolved crude platinum in aqua regia and found a second metal in the solution besides platinum. He named it after the asteroid Pallas, discovered only shortly before. At first he offered the metal anonymously in a London shop as "new silver", which caused a dispute among chemists until he revealed that he was the author.
Properties and reactions
Palladium is in period 5 and group 10, below nickel and above platinum. Its electron configuration is [Kr] 4d¹⁰ and its electronegativity is 2.2. The metal has a density of 12 g/cm³, melts at 1554.9 °C and boils at 2963 °C. It is soft, malleable and ductile.
Palladium is a precious metal, but less noble than platinum. It hardly tarnishes in air at room temperature. Nitric acid and hot concentrated sulfuric acid attack it, which platinum resists, and aqua regia dissolves it easily. The most common oxidation numbers in compounds are plus two and plus four.
Its most striking property is hydrogen absorption. Palladium absorbs up to about 900 times its own volume of hydrogen gas, as the hydrogen atoms settle between the metal atoms. The metal splits hydrogen molecules into single atoms, which are very reactive. That makes palladium an outstanding hydrogenation catalyst. Thin palladium walls also let only hydrogen pass through, which is used to purify the gas.
Uses in everyday life and technology
More than half of all palladium ends up in the catalytic converters of petrol cars. There it works with platinum and rhodium to turn toxic carbon monoxide and unburnt hydrocarbons into carbon dioxide and water. Further amounts go into electronics, for example contacts and capacitors in phones and computers.
In jewellery, palladium is a component of white gold: gold is alloyed with palladium and gets a pale colour. In dentistry, palladium-containing alloys are used for crowns and bridges.
In chemistry, palladium is one of the most important catalysts of all. With its help, carbon atoms from different molecules can be joined in a controlled way. These cross-coupling reactions, named after Richard Heck, Ei-ichi Negishi and Akira Suzuki, earned the three of them the Nobel Prize in Chemistry in 2010. They are used to make drugs, crop-protection agents and the light-emitting materials in OLED displays. Palladium on activated carbon is a common hydrogenation catalyst in laboratories and industry. Palladium is also studied and used as a hydrogen store and as a filter in hydrogen technology.
Quick question
What is the atomic number of palladium (Pd)?
FAQ
What is palladium used for?
Mainly for car catalytic converters, and also for electronics, dental alloys, white gold and as a catalyst in the chemical industry. Palladium catalysts join carbon atoms and so help to make drugs and plastics.
How much hydrogen can palladium absorb?
Palladium absorbs up to about 900 times its own volume of hydrogen. The hydrogen atoms sit between the palladium atoms, and the metal swells slightly. On heating or at low pressure it releases the hydrogen again.
What does palladium have to do with the Nobel Prize?
Richard Heck, Ei-ichi Negishi and Akira Suzuki won the 2010 Nobel Prize in Chemistry for palladium-catalysed cross-coupling. With palladium, carbon atoms from different molecules are joined to form larger molecules.