Nobelium at a glance
- Atomic number
- 102
- Symbol
- No
- Atomic mass
- 259 u
- Category
- Actinide
- Period
- 7
- Electron configuration
- [Rn] 5f 14 7s 2
- State at 25 °C
- Solid
- Melting point
- 826.9 °C
- Boiling point
- n/a
- Density
- 9.9 g/cm³
- Electronegativity
- 1.3
- 1st ionization energy
- 642 kJ/mol
- Oxidation states
- +2
- Discovered
- 1958
Atomic structure
A neutral nobelium atom has 102 protons and 102 electrons, and its most common isotope carries about 157 neutrons. One atom weighs about 4.3 × 10⁻²⁵ kg, so a mole of nobelium (6.022 × 10²³ atoms) weighs 259 g. That nucleus holds 1.54 neutrons for every proton, typical of heavy nuclei, which need extra neutrons to hold together against the protons' repulsion. The electrons fill 7 shells (K 2, L 8, M 18, N 32, O 32, P 8, Q 2), which gives the configuration [Rn] 5f 14 7s 2. As a f-block element, both its outer s electrons and its inner f electrons can take part in bonding, which is why it shows more than one oxidation state. All of its isotopes are radioactive, so any nobelium on Earth is either freshly made or part of a decay chain.
Physical properties
At room temperature (25 °C) nobelium is a solid. It melts at about 826.9 °C; its boiling point has not been measured. It stays solid in a hot kitchen oven but would melt in a candle flame (about 1000 °C). Its density is 9.9 g/cm³ (predicted), 9.9 times as dense as water.
Chemical behaviour
Its electronegativity is 1.3 on the Pauling scale. Pulling off its outermost electron takes 642 kJ/mol. A bond between nobelium and chlorine would be mostly ionic, with an electronegativity difference of 1.9. Its common oxidation states are +2, so in compounds you will usually meet it as No²⁺. It shares much of its chemistry with the other actinides, especially its neighbours Fermium, Mendelevium and Lawrencium. Handle with care: as a pure element it is radioactive.
Where it comes from
It does not occur in nature in useful amounts; it is made in nuclear reactors or particle accelerators. It was first made at Berkeley and at Dubna, Russia, after a long priority dispute by fusing curium with carbon nuclei. Its longest-lived known isotope is No-259, about 58 minutes. It was first identified in 1958, making it the 102nd element to be discovered (counting the 11 known since antiquity together).