Curium

Curium (Cm) is an actinide with atomic number 96. It has no stable isotope and no characteristic isotopic composition on Earth, so it has no standard atomic weight; its longest-lived isotope has mass number 247. Its electron configuration is [Rn] 5f7 6d1 7s2.

Curium key data

Symbol	Cm
Atomic number	96
Mass number (longest-lived isotope)	247
Protons	96
Neutrons (longest-lived isotope, curium-247)	151
Electrons (neutral atom)	96
Category	Actinide
Period	7
Group	f-block, no group
Block	f-block
Noble gas notation	[Rn] 5f7 6d1 7s2
Full electron configuration	1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f7 6s2 6p6 6d1 7s2
Electrons per shell	2, 8, 18, 32, 25, 9, 2

Mass number: IUPAC CIAAW radioactive elements, from NUBASE2020.

Measured properties of curium

From PubChem, the US National Institutes of Health. Public domain. PubChem’s periodic table gives no value for the rows not shown. The first ionisation energy is the value for curium from the NIST Atomic Spectra Database, in place of PubChem’s 6.02 eV.

Melting point	1618 K (1344.9 °C), 36th of 103 by melting point
Boiling point	3400 K (3126.9 °C), 31st of 93 by boiling point
Density	13.51 g/cm³, 15th of 96 by density
State at room temperature	Solid
Electronegativity	1.3 (Pauling), joint 61st of 95 by electronegativity
Van der Waals radius	245 pm, joint 9th of 99 by atomic radius
First ionisation energy	5.9922 eV (578.16 kJ/mol), 78th of 102 by first ionisation energy
Common oxidation states	+3
Discovered	1944

Each rank counts down from the highest value, among the elements PubChem’s table gives one for, and links to that property ranked across the whole table. All of them are collected under periodic trends.

Curium electron configuration

The full electron configuration of curium is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f7 6s2 6p6 6d1 7s2. With the noble-gas core collapsed it is [Rn] 5f7 6d1 7s2, and the outermost subshell is 7s2.

Curium does not follow the simple filling order. Its measured configuration differs from the one the aufbau rule predicts, because a half-filled 5f subshell sits lower in energy, so the electron that would be its eighth goes into 6d instead. This is a measured fact about the atom rather than an exception to be derived.

Bohr model of curium (Cm): a nucleus and seven shells holding 2, 8, 18, 32, 25, 9 and 2 electrons from the inside out. — Bohr model of curium (Cm). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Orbital box diagram of curium (Cm), [Rn] 5f7 6d1 7s2, with the radon core drawn as one bar: one box per orbital, filled by Hund’s rule, showing eight unpaired electrons, in 5f and 6d. — Orbital diagram of curium (Cm). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Why group and block follow from this is set out in how the periodic table maps electron configuration, and the electron configuration of every element is listed on one page.

Mass of one curium atom

A molar mass is grams per mole, so dividing by the Avogadro constant gives the mass of a single atom, or for an element with more than one natural isotope the average mass of its atoms, and the reciprocal gives how many atoms sit in a gram.

Mass of one atom

one atom = 247 / (6.02214076 × 10²³)
= 4.1015 × 10⁻²² g

Atoms in one gram

atoms per gram = 6.02214076 × 10²³ / 247
= 2.4381 × 10²¹ atoms

Curium has no standard atomic weight, because it has neither a stable isotope nor a characteristic isotopic composition on Earth. The figure used above is the mass number of curium-247, so treat these as nominal.

Curium on the periodic table

Curium is in period 7, outside the numbered groups, in the f-block.