Zirconium at a glance
- Atomic number
- 40
- Symbol
- Zr
- Atomic mass
- 91.224 u
- Category
- Transition metal
- Group / period
- 4 / 5
- Electron configuration
- [Kr] 4d 2 5s 2
- State at 25 °C
- Solid
- Melting point
- 1855 °C
- Boiling point
- 4409 °C
- Density
- 6.506 g/cm³
- Electronegativity
- 1.33
- 1st ionization energy
- 640 kJ/mol
- Oxidation states
- +4
- Discovered
- 1789
Atomic structure
A neutral zirconium atom has 40 protons and 40 electrons, and its most common isotope carries about 50 neutrons. One atom weighs about 1.51 × 10⁻²⁵ kg, so a mole of zirconium (6.022 × 10²³ atoms) weighs 91.224 g. That nucleus holds 1.25 neutrons for every proton. The electrons fill 5 shells (K 2, L 8, M 18, N 10, O 2), which gives the configuration [Kr] 4d 2 5s 2. 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. It has 5 stable isotopes: Zr-90, Zr-91, Zr-92, Zr-94 and Zr-96. No other element has a stable isotope with mass number 90, so Zr-90 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) zirconium is a solid. It melts at 1855 °C and boils at 4409 °C, so it stays liquid across a 2554-degree range. It stays solid in molten iron (1538 °C) but would melt in a welding arc. Its density is 6.506 g/cm³, 6.51 times as dense as water. Its atoms have a radius of about 155 pm (Yttrium next door is 180 pm). In the solid, its atoms pack into a hexagonal close-packed lattice.
Chemical behaviour
Its electronegativity is 1.33 on the Pauling scale. Pulling off its outermost electron takes 640 kJ/mol, less than Titanium above it (659 kJ/mol). A bond between zirconium and chlorine would be mostly ionic, with an electronegativity difference of 1.8. It releases 41 kJ/mol when it gains an extra electron. Its common oxidation states are +4, so in compounds you will usually meet it as Zr⁴⁺. Group 4 also holds Titanium, Hafnium and Rutherfordium, which share its outer-shell pattern.
Where it comes from
It makes up 165 mg/kg of Earth's crust, the 18th most abundant element there. It was first identified in 1789, making it the 27th element to be discovered (counting the 11 known since antiquity together).