Periodic Table


Atomic number: 95
Atomic weight: 243
Symbol: Am
Group number: (actinide)
Electronic configuration: [Rn].5f7.7s2


The lustre of freshly prepared americium metal is whiter and more silvery than plutonium or neptunium prepared in the same manner. It appears to be more malleable than uranium or neptunium and americium tarnishes slowly in dry air at room temperature. Americium is a radioactive rare earth metal which must be handled with care to avoid contact, since it is a heavy a and g emitter. It is named after America. The a activity of 241Am is about three times that of radium. Americium is available to qualified users in the UK and in the USA.

General information

Discoveror: Glenn Seaborg, Ralph James., L. Morgan, Albert Ghiorso
Date discovered: 1944
Discovered at: USA
Meaning of name: From the English word "America"

Physical data

Standard state: solid at 298 K
Colour: silvery white
Density of solid at ambient temperature/kg m-3: no data
Molar volume/cm3: 17.63

Radii /pm

Atomic: no data
Covalent (single bond):
Pauling radius for the ion [Am]-: no data

Valence shell orbital radius maxima (Rmax)
orbital s p d f
radius228.5no datano data48.1


Both values are quoted on the Pauling scale.

Pauling: 1.3
Allred Rochow: 1.2

Crystal Structure

structure: hcp (hexagonal close-packed)

The following CrystalMaker image represents the solid state structure. For most elements, the most stable allotrope is illustrated. Try WebElements version 2 for interactive virtual reality and CHIME images.


Temperatures (/K)

melting point: 1449
boiling point: 273

Enthalpies /kJ mol-1

fusion: 14.4
vaporization: no data

single bond enthalpies:
Am-F Am-Cl Am-Br Am-I Am-Am
no data no data no data 318 no data

Ionization enthalpies /kJ mol-1

Number Enthalpy


This section gives some data for naturally occurring isotopes.
Nominal mass Accurate mass % natural abundance

Further Information

Copyright 1997 Mark Winter
Department of Chemistry at the University of Sheffield, Sheffield S3 7HF, England.

The current version of this document is at http://www.shef.ac.uk/~chem/web-elements-I/Am.html