Platinum at a glance
- Atomic number
- 78
- Symbol
- Pt
- Atomic mass
- 195.08 u
- Category
- Transition metal
- Group / period
- 10 / 6
- Electron configuration
- [Xe] 4f 14 5d 9 6s 1
- State at 25 °C
- Solid
- Melting point
- 1768 °C
- Boiling point
- 3825 °C
- Density
- 21.45 g/cm³
- Electronegativity
- 2.28
- 1st ionization energy
- 870 kJ/mol
- Oxidation states
- +2, +4
- Discovered
- 1735
Atomic structure
A neutral platinum atom has 78 protons and 78 electrons, and its most common isotope carries about 117 neutrons. One atom weighs about 3.24 × 10⁻²⁵ kg, so a mole of platinum (6.022 × 10²³ atoms) weighs 195.08 g. That nucleus holds 1.5 neutrons for every proton, typical of heavy nuclei, which need extra neutrons to hold together against the protons' repulsion. The electrons fill 6 shells (K 2, L 8, M 18, N 32, O 17, P 1), which gives the configuration [Xe] 4f 14 5d 9 6s 1. 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 5 stable isotopes: Pt-192, Pt-194, Pt-195, Pt-196 and Pt-198. No other element has a stable isotope with mass number 195, so Pt-195 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) platinum is a solid. It melts at 1768 °C and boils at 3825 °C, so it stays liquid across a 2057-degree range. It stays solid in molten iron (1538 °C) but would melt in a welding arc. Its density is 21.45 g/cm³, 21.4 times as dense as water, the 10th densest element. Its atoms have a radius of about 135 pm (Iridium next door is 135 pm). In the solid, its atoms pack into a face-centred cubic lattice.
Chemical behaviour
Its electronegativity is 2.28 on the Pauling scale. Pulling off its outermost electron takes 870 kJ/mol, more than Palladium above it (804 kJ/mol). A bond between platinum and chlorine would be polar covalent, with an electronegativity difference of 0.88. It releases 205 kJ/mol when it gains an extra electron. Its common oxidation states are +2, +4, so in compounds you will usually meet it as Pt²⁺ and Pt⁴⁺. Group 10 also holds Nickel, Palladium and Darmstadtium, which share its outer-shell pattern.
Where it comes from
It makes up 0.005 mg/kg of Earth's crust, the 74th most abundant element there. It was first identified in 1735, making it the 15th element to be discovered (counting the 11 known since antiquity together).