Samarium at a glance
- Atomic number
- 62
- Symbol
- Sm
- Atomic mass
- 150.36 u
- Category
- Lanthanide
- Period
- 6
- Electron configuration
- [Xe] 4f 6 6s 2
- State at 25 °C
- Solid
- Melting point
- 1072 °C
- Boiling point
- 1794 °C
- Density
- 7.52 g/cm³
- Electronegativity
- 1.17
- 1st ionization energy
- 545 kJ/mol
- Oxidation states
- +3, +2
- Discovered
- 1879
Atomic structure
A neutral samarium atom has 62 protons and 62 electrons, and its most common isotope carries about 88 neutrons. One atom weighs about 2.5 × 10⁻²⁵ kg, so a mole of samarium (6.022 × 10²³ atoms) weighs 150.36 g. That nucleus holds 1.42 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 24, O 8, P 2), which gives the configuration [Xe] 4f 6 6s 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. It has 5 stable isotopes: Sm-144, Sm-149, Sm-150, Sm-152 and Sm-154. Sm-147 is radioactive but lasts billions of years. Its most common isotope, Sm-150, has the same mass number as Nd-150: isobars, nuclei of different elements with 150 nucleons each.
Physical properties
At room temperature (25 °C) samarium is a solid. It melts at 1072 °C and boils at 1794 °C, so it stays liquid across a 722-degree range. It stays solid in a candle flame (about 1000 °C) but would melt in molten iron (1538 °C). Its density is 7.52 g/cm³, 7.52 times as dense as water. Its atoms have a radius of about 185 pm (Promethium next door is 185 pm). In the solid, its atoms pack into a rhombohedral lattice.
Chemical behaviour
Its electronegativity is 1.17 on the Pauling scale. Pulling off its outermost electron takes 545 kJ/mol. A bond between samarium and chlorine would be mostly ionic, with an electronegativity difference of 2. Its common oxidation states are +3, +2, so in compounds you will usually meet it as Sm³⁺ and Sm²⁺. It shares much of its chemistry with the other lanthanides, especially its neighbours Neodymium, Promethium, Europium and Gadolinium.
Where it comes from
It makes up 7.05 mg/kg of Earth's crust, the 40th most abundant element there. It was first identified in 1879, making it the 68th element to be discovered (counting the 11 known since antiquity together).