Niobium at a glance
- Atomic number
- 41
- Symbol
- Nb
- Atomic mass
- 92.906 u
- Category
- Transition metal
- Group / period
- 5 / 5
- Electron configuration
- [Kr] 4d 4 5s 1
- State at 25 °C
- Solid
- Melting point
- 2477 °C
- Boiling point
- 4744 °C
- Density
- 8.57 g/cm³
- Electronegativity
- 1.6
- 1st ionization energy
- 652 kJ/mol
- Oxidation states
- +5, +3
- Discovered
- 1801
Atomic structure
A neutral niobium atom has 41 protons and 41 electrons, and its most common isotope carries about 52 neutrons. One atom weighs about 1.54 × 10⁻²⁵ kg, so a mole of niobium (6.022 × 10²³ atoms) weighs 92.906 g. That nucleus holds 1.27 neutrons for every proton. The electrons fill 5 shells (K 2, L 8, M 18, N 12, O 1), which gives the configuration [Kr] 4d 4 5s 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 only one stable isotope, Nb-93. No other element has a stable isotope with mass number 93, so Nb-93 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) niobium is a solid. It melts at 2477 °C and boils at 4744 °C, so it stays liquid across a 2267-degree range. It stays solid in molten iron (1538 °C) but would melt in a welding arc. That is the 7th highest melting point of any element. Its density is 8.57 g/cm³, 8.57 times as dense as water. Its atoms have a radius of about 145 pm (Zirconium next door is 155 pm). In the solid, its atoms pack into a body-centred cubic lattice.
Chemical behaviour
Its electronegativity is 1.6 on the Pauling scale. Pulling off its outermost electron takes 652 kJ/mol, more than Vanadium above it (651 kJ/mol). A bond between niobium and chlorine would be polar covalent, with an electronegativity difference of 1.6. It releases 86 kJ/mol when it gains an extra electron. Its common oxidation states are +5, +3, so in compounds you will usually meet it as Nb⁵⁺ and Nb³⁺. Group 5 also holds Vanadium, Tantalum and Dubnium, which share its outer-shell pattern.
Where it comes from
It makes up 20 mg/kg of Earth's crust, the 33rd most abundant element there. It was first identified in 1801, making it the 35th element to be discovered (counting the 11 known since antiquity together).