Palladium (Pd)

Palladium absorbs up to about 900 times its own volume of hydrogen at room temperature, swelling as the gas dissolves into the metal lattice — a property discovered by Thomas Graham in 1866 and still used to purify hydrogen, since nothing else diffuses through. It is the only element with no electrons in its outermost s shell in the ground state: [Kr] 4d¹⁰, with the 5s subshell completely empty, which no other atom manages. The 2010 Nobel Prize in Chemistry went to palladium-catalysed cross-coupling, the reaction family behind a large share of modern pharmaceuticals.

Properties

Atomic number

46

Symbol

Pd

Standard atomic weight

106.42 u

Category

Transition metal

Group

10

Period

5

Block

d-block

Electron configuration

[Kr] 4d¹⁰

Electrons per shell

2, 8, 18, 18

Melting point

1,828.05 K — 1554.9 °C — 2830.8 °F

Boiling point

3,236 K — 2962.8 °C — 5365.1 °F

Density

12 g/cm³

Electronegativity

2.2 (Pauling)

Oxidation states

+2, +4

Discovered

1802 by William Hyde Wollaston

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 4d¹⁰

Noble gas shorthand

[Kr] 4d¹⁰

Shells

K: 2 · L: 8 · M: 18 · N: 18

What it is used for

• Catalytic converters on petrol engines
• Multilayer ceramic capacitors, in enormous numbers
• Palladium-catalysed cross-coupling in drug manufacture
• Hydrogen purification membranes
• White gold alloys and dental crowns

The most striking configuration anomaly in the table: [Kr] 4d¹⁰, with an empty 5s subshell, rather than the predicted 5s² 4d⁸. Palladium is the only element whose ground state has no electrons in the outermost s orbital.

Neighbours in the table

Group 10

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Palladium sits in group 10 with nickel, platinum, darmstadtium.