Americium is a radioactive element familiar through its use in some smoke alarms. It can be produced through nuclear transformations involving plutonium and occurs only in traces in natural materials. Americium connects an everyday sensing device to isotope production and the detection of radiation.
FROM ELEMENT TO APPLICATION
AN ELEMENT IN CONTEXT
How can smoke change a current maintained by an ionizing source?
Americium-241 in sealed ionization sources
FOLLOW THIS RELATIONSHIP
Establish the clear-air signal
An americium-241 source ionizes air inside the detector chamber. Electrodes collect charged species, establishing an electrical signal that the detector circuitry can monitor.
Explore the model (optional)
AmFORM → INTERACTION → CONSEQUENCE
Follow the explanation in either direction. These are authored steps, not a physical clock.
Optional: check your understanding
USE THE EXPLANATION
Which signal is central to this detection mechanism?
Atomic identity, reference properties and structure models
INTERACTIVE 3D95 visible sites
An electron censusDrag to rotate · scroll or pinch to zoom
[Rn] 5f7 7s2. Grouping electrons by principal shell connects the neutral-atom configuration to periodic-table patterns.
What this representation means
A three-dimensional counting diagram. Dots are count markers, not measured electron positions; spherical placement and spacing are arbitrary. There are no electron trajectories here. Orbitals describe quantum states.
Americium was discovered in 1944 by a team working in the wartime transuranium program. Glenn Seaborg’s actinide concept helped explain why these heavy elements should form a second inner-transition series rather than simply continue ordinary transition-metal chemistry. Named for the Americas, americium echoed europium in the lighter series. Its later appearance in ionization smoke gave an artificial element a surprisingly familiar, carefully engineered application.
1944
Discovery
Seaborg and colleagues identify americium in the transuranium program.
1945
Public announcement
The discovery becomes public after wartime secrecy.
Americium is principally manufactured or formed as other artificial actinides . Americium-241 can accumulate in material containing plutonium-241, linking its availability to an evolving inventory. There is no ordinary americium supply. Samples must be described by isotope and chemical form rather than by the element name alone.
Americium commonly forms +3 compounds, making comparisons with trivalent lanthanides useful while leaving important actinide differences to investigate. Its radioactive decay continues inside those compounds. A neutral-atom configuration describes an isolated atom, whereas a source, or dissolved has a different chemical environment and electronic description.
Each section links its supporting references. Descriptions and diagrams were authored for this atlas. Source checks completed. Source checks dated 2026-09-06.
CITE THIS PAGE
Family labels and the group-3 layout are teaching conventions. A table position does not establish an unknown bulk property. Electronic configurations refer to neutral atoms; ions and material environments require their own account.
Read each reference property together with its units, conditions and the reference information below.
Not yet recorded: electron affinity, second ionization energy, ground-state term symbol, van der waals radius, thermal conductivity, specific heat capacity, electrical resistivity, crystal structure, lattice parameter a, space group, abundance in seawater, abundance in the universe, annual production, supply-risk listing, hazard classification, biological role, cas registry number, pubchem cid, wikidata item. These families are declared and awaiting an ingestion run against the source each names; they are listed here rather than shown as empty rows.
Physical
Intended reference: CRC Handbook of Chemistry and Physics · CRC Press
Phase at 20 °C
solid
293.15 K, 101.325 kPa
Melting point in K
1449 K
at standard pressure unless noted
Boiling point in K
2880 K
101.325 kPa
Density in g/cm³
13.69 g/cm³
solids and liquids at 293 K; gases at 273 K and 101.325 kPa
Atomic and electronic
Intended reference: Pauling electronegativity scale · Widely tabulated; original in The Nature of the Chemical Bond
Electronegativity in Pauling
1.13 Pauling
Pauling scale
First ionization energy in kJ/mol
578 kJ/mol
gas-phase neutral atom, ground state
Covalent radius in pm
180 pm
single-bond covalent radius
Common oxidation states
+3
states met in ordinary compounds; not exhaustive
Occurrence and identity
Intended reference: CRC Handbook of Chemistry and Physics · CRC Press
Abundance in the crust
Not recorded
upper continental crust, mass fraction
Has stable isotopes
Noderived
Discovery year in CE
1944 CEderived
WHERE IT SITS AMONG ITS NEIGHBORS
Electronegativity across period 7
Am 1.13 Pauling · Americium
Electronegativity across period 7. Exact values.
Ra
0.9
Ac
1.1
Th
1.3
Pa
1.5
U
1.38
Np
1.36
Pu
1.28
Am
1.13
Cm
1.28
Bk
1.3
Cf
1.3
Es
1.3
Fm
1.3
Md
1.3
No
1.3
Lr
1.3
The trend uses the same reference-property values as the periodic table. Elements with no recorded value are left out of the line rather than drawn at zero.
