Hafnium at a glance
- Atomic number
- 72
- Symbol
- Hf
- Atomic mass
- 178.49 u
- Category
- Transition metal
- Group / period
- 4 / 6
- Electron configuration
- [Xe] 4f 14 5d 2 6s 2
- State at 25 °C
- Solid
- Melting point
- 2233 °C
- Boiling point
- 4603 °C
- Density
- 13.31 g/cm³
- Electronegativity
- 1.3
- 1st ionization energy
- 659 kJ/mol
- Oxidation states
- +4
- Discovered
- 1923
Atomic structure
A neutral hafnium atom has 72 protons and 72 electrons, and its most common isotope carries about 106 neutrons. One atom weighs about 2.96 × 10⁻²⁵ kg, so a mole of hafnium (6.022 × 10²³ atoms) weighs 178.49 g. That nucleus holds 1.47 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 10, P 2), which gives the configuration [Xe] 4f 14 5d 2 6s 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: Hf-176, Hf-177, Hf-178, Hf-179 and Hf-180. No other element has a stable isotope with mass number 178, so Hf-178 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) hafnium is a solid. It melts at 2233 °C and boils at 4603 °C, so it stays liquid across a 2370-degree range. It stays solid in molten iron (1538 °C) but would melt in a welding arc. That is the 10th highest melting point of any element. Its density is 13.31 g/cm³, 13.3 times as dense as water. Its atoms have a radius of about 155 pm (Lutetium next door is 175 pm). In the solid, its atoms pack into a hexagonal close-packed lattice.
Chemical behaviour
Its electronegativity is 1.3 on the Pauling scale. Pulling off its outermost electron takes 659 kJ/mol, more than Zirconium above it (640 kJ/mol). A bond between hafnium and chlorine would be mostly ionic, with an electronegativity difference of 1.9. It releases 17 kJ/mol when it gains an extra electron. Its common oxidation states are +4, so in compounds you will usually meet it as Hf⁴⁺. Group 4 also holds Titanium, Zirconium and Rutherfordium, which share its outer-shell pattern.
Where it comes from
It makes up 3 mg/kg of Earth's crust, the 47th most abundant element there. It was first identified in 1923, making it the 87th element to be discovered (counting the 11 known since antiquity together).