Lawrencium

Lawrencium (Lr) is an actinide with atomic number 103. It has no stable isotope and no characteristic isotopic composition on Earth, so it has no standard atomic weight. CIAAW lists lawrencium-262 and lawrencium-266 as its longest-lived isotopes, and the mass number used here is 266. Its electron configuration is [Rn] 5f14 7s2 7p1.

Lawrencium key data

Symbol	Lr
Atomic number	103
Mass number (one of its longest-lived isotopes)	266
Protons	103
Neutrons (lawrencium-266)	163
Electrons (neutral atom)	103
Category	Actinide
Period	7
Group	3 or none, by convention
Block	d-block or f-block, by convention
Noble gas notation	[Rn] 5f14 7s2 7p1
Full electron configuration	1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f14 6s2 6p6 7s2 7p1
Electrons per shell	2, 8, 18, 32, 32, 8, 3

Mass number: IUPAC CIAAW radioactive elements, from NUBASE2020.

Properties of lawrencium

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

Predicted melting point	1900 K (1626.9 °C), 21st of 103 by melting point
Predicted state at room temperature	Solid
Predicted electronegativity	1.3 (Pauling), joint 61st of 95 by electronegativity
Common oxidation states	+3
Discovered	1961

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.

Lawrencium electron configuration

The full electron configuration of lawrencium is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f14 6s2 6p6 7s2 7p1. With the noble-gas core collapsed it is [Rn] 5f14 7s2 7p1, and the outermost subshell is 7p1.

Lawrencium does not follow the simple filling order. The aufbau rule puts its last electron in 6d, but in an atom this heavy, relativistic effects stabilise the 7p subshell, and calculations place that electron in 7p instead. A 2015 measurement of its first ionisation energy supports that configuration.

Bohr model of lawrencium (Lr): a nucleus and seven shells holding 2, 8, 18, 32, 32, 8 and 3 electrons from the inside out. — Bohr model of lawrencium (Lr). 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 lawrencium (Lr), [Rn] 5f14 7s2 7p1, with the radon core drawn as one bar: one box per orbital, filled by Hund’s rule, showing one unpaired electron, in 7p. — Orbital diagram of lawrencium (Lr). 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 lawrencium 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 = 266 / (6.02214076 × 10²³)
= 4.4170 × 10⁻²² g

Atoms in one gram

atoms per gram = 6.02214076 × 10²³ / 266
= 2.2640 × 10²¹ atoms

Lawrencium 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 lawrencium-266, so treat these as nominal.

Lawrencium on the periodic table

Lawrencium is in period 7. Tables differ on its group: some place it in group 3, in the d-block, and others in the f-block, outside the numbered groups.