Palladium at a glance
- Atomic number
- 46
- Symbol
- Pd
- Atomic mass
- 106.42 u
- Category
- Transition metal
- Group / period
- 10 / 5
- Electron configuration
- [Kr] 4d 10
- State at 25 °C
- Solid
- Melting point
- 1555 °C
- Boiling point
- 2963 °C
- Density
- 12.02 g/cm³
- Electronegativity
- 2.2
- 1st ionization energy
- 804 kJ/mol
- Oxidation states
- +2, +4
- Discovered
- 1803
Atomic structure
A neutral palladium atom has 46 protons and 46 electrons, and its most common isotope carries about 60 neutrons. One atom weighs about 1.77 × 10⁻²⁵ kg, so a mole of palladium (6.022 × 10²³ atoms) weighs 106.42 g. That nucleus holds 1.3 neutrons for every proton. The electrons fill 4 shells (K 2, L 8, M 18, N 18), which gives the configuration [Kr] 4d 10. As a d-block element, both its outer s electrons and its inner d electrons can take part in bonding, which is why it shows more than one oxidation state. Its configuration breaks the simple filling order: one electron moves so that a subshell becomes half-full or full, which is more stable. It has 6 stable isotopes: Pd-102, Pd-104, Pd-105, Pd-106, Pd-108 and Pd-110. Its most common isotope, Pd-106, has the same mass number as Cd-106: isobars, nuclei of different elements with 106 nucleons each.
Physical properties
At room temperature (25 °C) palladium is a solid. It melts at 1555 °C and boils at 2963 °C, so it stays liquid across a 1408-degree range. It stays solid in molten iron (1538 °C) but would melt in a welding arc. Its density is 12.02 g/cm³, 12 times as dense as water. Its atoms have a radius of about 140 pm (Rhodium next door is 135 pm). In the solid, its atoms pack into a face-centred cubic lattice.
Chemical behaviour
Its electronegativity is 2.2 on the Pauling scale. Pulling off its outermost electron takes 804 kJ/mol, more than Nickel above it (737 kJ/mol). A bond between palladium and chlorine would be polar covalent, with an electronegativity difference of 0.96. It releases 54 kJ/mol when it gains an extra electron. Its common oxidation states are +2, +4, so in compounds you will usually meet it as Pd²⁺ and Pd⁴⁺. Group 10 also holds Nickel, Platinum and Darmstadtium, which share its outer-shell pattern.
Where it comes from
It makes up 0.015 mg/kg of Earth's crust, the 70th most abundant element there. It was first identified in 1803, making it the 38th element to be discovered (counting the 11 known since antiquity together).