Some elements exist in several different structural forms, called allotropes. Each allotrope has different physical properties.

For more information on the Visual Elements image see the Uses and properties section below.



A vertical column in the periodic table. Members of a group typically have similar properties and electron configurations in their outer shell.

A horizontal row in the periodic table. The atomic number of each element increases by one, reading from left to right.

Elements are organised into blocks by the orbital type in which the outer electrons are found. These blocks are named for the characteristic spectra they produce: sharp (s), principal (p), diffuse (d), and fundamental (f).

Atomic number
The number of protons in an atom.

Electron configuration
The arrangements of electrons above the last (closed shell) noble gas.

Melting point
The temperature at which the solid–liquid phase change occurs.

Boiling point
The temperature at which the liquid–gas phase change occurs.

The transition of a substance directly from the solid to the gas phase without passing through a liquid phase.

Density (g cm−3)
Density is the mass of a substance that would fill 1 cm3 at room temperature.

Relative atomic mass
The mass of an atom relative to that of carbon-12. This is approximately the sum of the number of protons and neutrons in the nucleus. Where more than one isotope exists, the value given is the abundance weighted average.

Atoms of the same element with different numbers of neutrons.

CAS number
The Chemical Abstracts Service registry number is a unique identifier of a particular chemical, designed to prevent confusion arising from different languages and naming systems.

Fact box

Group Actinides  Melting point 1176°C, 2149°F, 1449 K 
Period Boiling point 2011°C, 3652°F, 2284 K 
Block Density (g cm−3) 12 
Atomic number 95  Relative atomic mass [243]  
State at 20°C Solid  Key isotopes 241Am, 243Am 
Electron configuration [Rn] 5f77s2  CAS number 7440-35-9 
ChemSpider ID 22405 ChemSpider is a free chemical structure database


Image explanation

Murray Robertson is the artist behind the images which make up Visual Elements. This is where the artist explains his interpretation of the element and the science behind the picture.


The description of the element in its natural form.

Biological role

The role of the element in humans, animals and plants.

Natural abundance

Where the element is most commonly found in nature, and how it is sourced commercially.

Uses and properties

Image explanation
The image reflects both the origin of the element’s name and its presence in domestic smoke alarms.
Americium is a silvery, shiny radioactive metal.
Americium is commonly used in smoke alarms, but has few other uses.

It has the potential to be used in spacecraft batteries in the future. Currently plutonium is used but availability is poor so alternatives are being considered.

It is of interest as part of the decay sequence that occurs in nuclear power production.
Biological role
Americium has no known biological role. It is toxic due to its radioactivity.
Natural abundance
Americium occurs naturally in uranium minerals, but only in trace amounts. The main source of the element is the neutron bombardment of plutonium in nuclear reactors. A few grams are produced in this way each year.

It is also formed when nuclear weapons are detonated.
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This element was in fact discovered after curium, the element which follows it in the periodic table. However, it did once exist on Earth having been produced for millions of years in natural nuclear reactors in Oklo, Gabon. These ceased to function a billion years ago, and as the longest lived isotope is americium-247, with a half-life of 7370 years, none has survived to the present day. Americium was first made late in 1944 at the University of Chicago by a team which included Glenn Seaborg, Ralph James, Leon Morgan, and Albert Ghiorso. The americium was produced by bombarding plutonium with neutrons in a nuclear reactor. This produced isotope americium-241, which has a half-life of this is 432 years.

Atomic radius, non-bonded
Half of the distance between two unbonded atoms of the same element when the electrostatic forces are balanced. These values were determined using several different methods.

Covalent radius
Half of the distance between two atoms within a single covalent bond. Values are given for typical oxidation number and coordination.

Electron affinity
The energy released when an electron is added to the neutral atom and a negative ion is formed.

Electronegativity (Pauling scale)
The tendency of an atom to attract electrons towards itself, expressed on a relative scale.

First ionisation energy
The minimum energy required to remove an electron from a neutral atom in its ground state.

Atomic data

Atomic radius, non-bonded (Å) 2.44 Covalent radius (Å) 1.73
Electron affinity (kJ mol−1) Unknown Electronegativity
(Pauling scale)
Ionisation energies
(kJ mol−1)


Common oxidation states

The oxidation state of an atom is a measure of the degree of oxidation of an atom. It is defined as being the charge that an atom would have if all bonds were ionic. Uncombined elements have an oxidation state of 0. The sum of the oxidation states within a compound or ion must equal the overall charge.


Atoms of the same element with different numbers of neutrons.

Key for isotopes

Half life
  y years
  d days
  h hours
  m minutes
  s seconds
Mode of decay
  α alpha particle emission
  β negative beta (electron) emission
  β+ positron emission
  EC orbital electron capture
  sf spontaneous fission
  ββ double beta emission
  ECEC double orbital electron capture

Oxidation states and isotopes

Common oxidation states 6, 5, 4, 3
Isotopes Isotope Atomic mass Natural abundance (%) Half life Mode of decay
  241Am 241.057 - 432.7 y  α 
        1.2 x 1014 sf 
  243Am 243.061 - 7.37 x 103 α 
        2 x 1014 sf 


Data for this section been provided by the British Geological Survey.

Relative supply risk

An integrated supply risk index from 1 (very low risk) to 10 (very high risk). This is calculated by combining the scores for crustal abundance, reserve distribution, production concentration, substitutability, recycling rate and political stability scores.

Crustal abundance (ppm)

The number of atoms of the element per 1 million atoms of the Earth’s crust.

Recycling rate

The percentage of a commodity which is recycled. A higher recycling rate may reduce risk to supply.


The availability of suitable substitutes for a given commodity.
High = substitution not possible or very difficult.
Medium = substitution is possible but there may be an economic and/or performance impact
Low = substitution is possible with little or no economic and/or performance impact

Production concentration

The percentage of an element produced in the top producing country. The higher the value, the larger risk there is to supply.

Reserve distribution

The percentage of the world reserves located in the country with the largest reserves. The higher the value, the larger risk there is to supply.

Political stability of top producer

A percentile rank for the political stability of the top producing country, derived from World Bank governance indicators.

Political stability of top reserve holder

A percentile rank for the political stability of the country with the largest reserves, derived from World Bank governance indicators.



Specific heat capacity (J kg−1 K−1)

Specific heat capacity is the amount of energy needed to change the temperature of a kilogram of a substance by 1 K.

Young's modulus

A measure of the stiffness of a substance. It provides a measure of how difficult it is to extend a material, with a value given by the ratio of tensile strength to tensile strain.

Shear modulus

A measure of how difficult it is to deform a material. It is given by the ratio of the shear stress to the shear strain.

Bulk modulus

A measure of how difficult it is to compress a substance. It is given by the ratio of the pressure on a body to the fractional decrease in volume.

Vapour pressure

A measure of the propensity of a substance to evaporate. It is defined as the equilibrium pressure exerted by the gas produced above a substance in a closed system.

Pressure and temperature data – advanced

Specific heat capacity
(J kg−1 K−1)
Unknown Young's modulus (GPa) Unknown
Shear modulus (GPa) Unknown Bulk modulus (GPa) Unknown
Vapour pressure  
Temperature (K)
400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
Pressure (Pa)
- - 3.88
x 10-7
0.00167 0.423 21.35 - - - - -
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Listen to Americium Podcast
Transcript :

Chemistry in its element: americium


You're listening to Chemistry in its element brought to you by Chemistry World, the magazine of the Royal Society of Chemistry.

(End promo)

Chris Smith

Elementary envy tops the bill this week with a substance that was christened to compete with Europium. It was announced to the world via the slightly unorthodox route of a kids' radio show, but this stuff is none the less worth its weight in gold, in fact its worth more than that, 60 times as much in fact, because its gone on to save thousands of lives and homes around the world since. And to tell us how here's Brian Clegg:

Brian Clegg

Keeping up with the neighbours is rarely a concern in the periodic table. Nitrogen doesn't care much what carbon and oxygen are up to, and rarely casts covetous glances at phosphorous. But there's at least one substance in the periodic table that was named in response to a nearby element, and that's americium, the element that looks as if it should be pronounced [AMER-ICK-IUM].

It sits in the seventh position in the actinides, those mostly artificial substances that inhabit the second of the periodic table's floating bars of elements, and directly above it, in the parallel list of lanthanides, you will find europium. Americium's name, according to its discoverers, is 'suggested on the basis of its position. analogous to europium' - but let's face it, you could equally blame its name on continent envy. However it was dreamed up, it's an improvement on the provisional names given to americium and curium (discovered at the same time) - originally they were pandemonium and delirium.

Americium didn't exist until Glen T. Seaborg and his colleagues, working on the Manhattan Project in the Metallurgical Laboratory at the University of Chicago, produced it in 1944. It feels strange to say that Seaborg took out a patent on this 'element 95'. Seaborg's team would isolate a total of 10 new elements, re-arranging the structure of the periodic table.

The first hint the world had of the existence of americium came not in a paper for a distinguished journal, but on a children's radio quiz in 1945. Seaborg appeared as a guest on NBC's Quiz Kids show, where one of the participants asked him if they had produced any other new elements as well as plutonium and neptunium. As Seaborg was due to formally announce the discovery of americium five days later, he let slip its existence, along with element 96, later called curium.

The first isotope of americium produced was americium 241, still the most commonly used form. The Manhattan Project was busy creating plutonium to be used in nuclear weapons, and some plutonium 239 went through a process of capturing extra neutrons to become 240 and then 241, which gave off an electron from the nucleus to turn into americium.

None of americium's isotopes are truly stable - the longest lasting, americium-243, has a half-life of 7370 years, while many of the 18 isotopes produced only hang around for minutes. Like many of the actinides, Americium is silver-white in appearance, and reasonably heavy with a density similar to that of lead. It's a solid at room temperature - you'd need to heat it to over 1,000 degrees Celsius to melt it.

But americium has one unique quality. It's the only artificial element - and the only radioisotope - that is routinely found in the home.

Actually, I ought to qualify that. We all have traces of natural radioactive elements in our houses. If you live somewhere like Cornwall with a high preponderance of granite, you will have more than your fair share, for instance, of radon about the place, giving a background radiation level of three times that experienced in London. But americium is the only radioisotope you are likely to go down to the supermarket and buy - what's more, you will have been encouraged to do so by the government.

That's because americium is used in many smoke detectors. A tiny quantity - less than a millionth of a gram - of americium 241 oxide will be sitting in there, beaming out radiation as it slowly transforms to neptunium with a half life of 432 years. The alpha particles flowing from the americium (it's a better alpha source than radium) pass through a small compartment where they ionize the air, allowing a tiny electrical current to cross the chamber. If smoke particles get in there, they absorb the alpha particles before they can create ions, stopping the current flowing and setting off the alarm.

Every now and then someone will panic when they discover that not only is there a radioactive material in household smoke detectors, but it could, in principle, be used to produce a nuclear weapon. Assemble enough of that americium-241 and it would go critical. But before any terrorist groups try to corner the market in smoke detectors it's worth pointing out that it would take around 180 billion of them to have sufficient americium-241 assembled to go critical - and even then it wouldn't be enough to put the detectors together in the same place, you would have to painstakingly extract each of those 180 billion specks of the element and mould them together, an effort that would take thousands of years.

Americium has also found other uses for its radioactive emissions, as a source of both alpha particles and gamma rays for medical applications and in industry - but its use is limited to jobs where only a small quantity is required, as it is expensive to produce. The americium oxide used in smoke detectors costs around $1500 per gram - compare this with the current gold price of around $30 per gram. There's a nice irony that the element named after the world's richest, most consumption-oriented nation is only typically used in very small quantities.

One of my favourite books in my youth was Isaac Asimov's Foundation. In this book, tiny, walnut sized atomic generators are commonplace. This was one of science fiction's dreams that never came true - and many people would now be horrified at the thought of personal use of nuclear power. Yet this one element, americium, is the radioactive heart that helps keep our homes safe.

Chris Smith

Brian Clegg with the element that was born out of envy, but I'm not sure if its compounds are green though. But next week's element certainly is, the discoverer named Thallium after the Greek word thallos, meaning "green shoot". But don't get distracted by its colour, because this stuff is deadly, sufficiently nasty in fact for Agatha Christie to have written a murder mystery about it.

Henry Nicholls

I slammed back the receiver, then took it off again. I dialled the number and was lucky enough this time to get Lejeune straight away. "Listen" I said, "is Ginger's hair coming out by the roots in handfuls?" "Well as a matter of fact I believe it is. High fever I suppose." "Fever my foot" I said "what Ginger's suffering from, what they've all suffered from is Thallium poisoning, please God may we be in time."

Chris Smith

And you can hear how Ginger gets on from Henry Nicholls on next week's Chemistry in its Element, do try and join us. I'm Chris Smith, thank you for listening and goodbye.


Chemistry in its element is brought to you by the Royal Society of Chemistry and produced by There's more information and other episodes of Chemistry in its element on our website at

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Visual Elements images and videos
© Murray Robertson 1998-2017.



W. M. Haynes, ed., CRC Handbook of Chemistry and Physics, CRC Press/Taylor and Francis, Boca Raton, FL, 95th Edition, Internet Version 2015, accessed December 2014.
Tables of Physical & Chemical Constants, Kaye & Laby Online, 16th edition, 1995. Version 1.0 (2005), accessed December 2014.
J. S. Coursey, D. J. Schwab, J. J. Tsai, and R. A. Dragoset, Atomic Weights and Isotopic Compositions (version 4.1), 2015, National Institute of Standards and Technology, Gaithersburg, MD, accessed November 2016.
T. L. Cottrell, The Strengths of Chemical Bonds, Butterworth, London, 1954.


Uses and properties

John Emsley, Nature’s Building Blocks: An A-Z Guide to the Elements, Oxford University Press, New York, 2nd Edition, 2011.
Thomas Jefferson National Accelerator Facility - Office of Science Education, It’s Elemental - The Periodic Table of Elements, accessed December 2014.
Periodic Table of Videos, accessed December 2014.


Supply risk data

Derived in part from material provided by the British Geological Survey © NERC.


History text

Elements 1-112, 114, 116 and 117 © John Emsley 2012. Elements 113, 115, 117 and 118 © Royal Society of Chemistry 2017.



Produced by The Naked Scientists.


Periodic Table of Videos

Created by video journalist Brady Haran working with chemists at The University of Nottingham.