Mendelevium at a glance
- Atomic number
- 101
- Symbol
- Md
- Atomic mass
- 258 u
- Category
- Actinide
- Period
- 7
- Electron configuration
- [Rn] 5f 13 7s 2
- State at 25 °C
- Solid
- Melting point
- 826.9 °C
- Boiling point
- n/a
- Density
- 10.3 g/cm³
- Electronegativity
- 1.3
- 1st ionization energy
- 635 kJ/mol
- Oxidation states
- +3
- Discovered
- 1955
Atomic structure
A neutral mendelevium atom has 101 protons and 101 electrons, and its most common isotope carries about 157 neutrons. One atom weighs about 4.28 × 10⁻²⁵ kg, so a mole of mendelevium (6.022 × 10²³ atoms) weighs 258 g. That nucleus holds 1.55 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 31, P 8, Q 2), which gives the configuration [Rn] 5f 13 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 mendelevium on Earth is either freshly made or part of a decay chain.
Physical properties
At room temperature (25 °C) mendelevium 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 10.3 g/cm³ (predicted), 10.3 times as dense as water.
Chemical behaviour
Its electronegativity is 1.3 on the Pauling scale. Pulling off its outermost electron takes 635 kJ/mol. A bond between mendelevium and chlorine would be mostly ionic, with an electronegativity difference of 1.9. Its common oxidation states are +3, so in compounds you will usually meet it as Md³⁺. It shares much of its chemistry with the other actinides, especially its neighbours Einsteinium, Fermium, Nobelium 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 by hitting a tiny sample of einsteinium-253 with alpha particles, one atom at a time. Its longest-lived known isotope is Md-258, about 51 days. It was first identified in 1955, making it the 101st element to be discovered (counting the 11 known since antiquity together).