Terbium

Terbium (Tb) is a lanthanide with atomic number 65 and an atomic mass of 158.93. Its electron configuration is [Xe] 4f9 6s2.

Terbium key data

Symbol	Tb
Atomic number	65
Atomic mass (standard atomic weight)	158.93
Molar mass	158.93 g/mol
Protons	65
Neutrons (most abundant isotope, terbium-159)	94
Electrons (neutral atom)	65
Category	Lanthanide
Period	6
Group	f-block, no group
Block	f-block
Noble gas notation	[Xe] 4f9 6s2
Full electron configuration	1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f9 5s2 5p6 6s2
Electrons per shell	2, 8, 18, 27, 8, 2

Atomic mass: IUPAC CIAAW abridged standard atomic weights, 2024. Most abundant isotope: NIST Atomic Weights and Isotopic Compositions.

Measured properties of terbium

From PubChem, the US National Institutes of Health. Public domain. PubChem’s periodic table gives no value for the rows not shown.

Melting point	1629 K (1355.9 °C), 35th of 103 by melting point
Boiling point	3503 K (3229.9 °C), 28th of 93 by boiling point
Density	8.23 g/cm³, 39th of 96 by density
State at room temperature	Solid
Van der Waals radius	221 pm, joint 32nd of 99 by atomic radius
First ionisation energy	5.864 eV (565.8 kJ/mol), 83rd of 102 by first ionisation energy
Common oxidation states	+3
Discovered	1843

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.

Terbium electron configuration

The full electron configuration of terbium is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f9 5s2 5p6 6s2. With the noble-gas core collapsed it is [Xe] 4f9 6s2, and the outermost subshell is 6s2.

Bohr model of terbium (Tb): a nucleus and six shells holding 2, 8, 18, 27, 8 and 2 electrons from the inside out. — Bohr model of terbium (Tb). 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 terbium (Tb), [Xe] 4f9 6s2, with the xenon core drawn as one bar: one box per orbital, filled by Hund’s rule, showing five unpaired electrons, in 4f. — Orbital diagram of terbium (Tb). 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 terbium 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 = 158.93 / (6.02214076 × 10²³)
= 2.6391 × 10⁻²² g

Atoms in one gram

atoms per gram = 6.02214076 × 10²³ / 158.93
= 3.7892 × 10²¹ atoms

Terbium on the periodic table

Terbium is in period 6, outside the numbered groups, in the f-block.