Americium (Am)
Element 95 · Actinide · atomic mass [243] u · f-block, period 7
Electron shells: 2, 8, 18, 32, 25, 8, 2
Americium is the one synthetic element most people own, sitting in a tiny amount inside ionisation smoke detectors. Its alpha particles ionise the air, and smoke interrupts the current.
There is a speck of a man-made element in most smoke alarms — about 0.3 micrograms of americium.
- Atomic number
- 95
- Atomic mass
- [243] u
- Electron configuration
- [Rn] 7s² 5f⁷
- Group · period · block
- f-block · 7 · f
- Oxidation states
- +3, +4, +5, +6
- Electronegativity
- 1.3 (Pauling)
- Melting point
- 1,176 °C
- Boiling point
- 2,011 °C
- Density
- 13.69 g/cm³
- State at 20 °C
- Solid
- Appearance
- Silvery-white metal
- Discovered
- Glenn T. Seaborg and colleagues, 1944
95 protons, and 95 electrons when neutral
Mass number of the longest-lived isotope — no standard atomic weight exists
Shown in the strip below the table
What Americium is used for
- Ionisation smoke detectors
- Industrial thickness gauges
- Neutron sources for well logging
Atomic structure of Americium
A neutral americium atom has 95 protons and 95 electrons. Its most common isotope carries about 148 neutrons, which is where the mass number 243 comes from. Change the neutron count and you have a different isotope of americium; change the proton count and you have a different element entirely.
Those electrons fill shells from the inside out: 2, 8, 18, 32, 25, 8, 2. In orbital notation that is [Rn] 7s² 5f⁷. As a f-block element its valence count is variable, which is why it can form compounds in several oxidation states.
3D electron orbitals
See where Americium's electrons actually are — a cloud of 26,000 points sampled from the probability density |ψ|², not a cartoon of a lobe.
Where Americium sits in the periodic table
Discovery and naming
Americium was discovered by Glenn T. Seaborg and colleagues in 1944. Its name comes from The Americas.
Every isotope of americium is radioactive, so it has no standard atomic weight. Periodic tables show the mass number of its longest-lived isotope in square brackets instead.
Keep exploring
Atomic weight from IUPAC/CIAAW; physical properties from PubChem (US National Library of Medicine). Values that have never been measured are shown as “not measured” rather than estimated.
Related Calculators
Frequently Asked Questions
What is the atomic mass of Americium?
Americium has no stable isotope, so it has no standard atomic weight. Periodic tables instead show 243 in square brackets — the mass number of its longest-lived isotope.
How many protons, neutrons and electrons does Americium have?
Americium has 95 protons, because the atomic number is the proton count and that is what makes an atom Americium at all. A neutral Americium atom has the same number of electrons, 95. The neutron count varies by isotope; the most common one has about 148, since 243 − 95 = 148.
What is the electron configuration of Americium?
The electron configuration of Americium is [Rn] 7s² 5f⁷. Its electrons are arranged 2, 8, 18, 32, 25, 8, 2 across 7 shells.
Is Americium a metal?
Yes. Americium is classified as a actinide, on the metallic side of the table.
What group and period is Americium in?
Americium sits in the f-block strip below the main table, in period 7, among the actinides. Elements in that strip are not given a group number in the standard 18-column layout.
What is the charge of Americium?
Americium most commonly forms a 3+ ion. Like most transition metals it has several accessible oxidation states, so the charge depends on the compound. Across all its compounds it shows oxidation states of +3, +4, +5, +6.
What is the melting point of Americium?
Americium melts at 1,176 °C and boils at 2,011 °C. That puts it in the solids at everyday temperatures.
What is Americium used for?
Ionisation smoke detectors, Industrial thickness gauges, Neutron sources for well logging. Americium is the one synthetic element most people own, sitting in a tiny amount inside ionisation smoke detectors.
Who discovered Americium, and where does its name come from?
Americium (Am) was discovered by Glenn T. Seaborg and colleagues in 1944. The name comes from The Americas.
Is Americium radioactive?
Yes. Every isotope of Americium is radioactive — it has no stable form at all, which is why the periodic table shows its mass in square brackets rather than as a standard atomic weight.