This chart shows (most) of the 3000+ known isotopes of the elements, organised by their number of protons (x-axis) and number of neutrons (y-axis).
The "stable isotopes" are coloured in green; these isotopes will never undergo nuclear decay.
Above the green line (too many neutrons)
- "Beta-minus decay": A neutron emits a beta-minus particle (a fast-moving electron) to form a proton. This causes the neutron number to decrease by one, and the proton number to increase by one, so the atom changes element; the mass number is unchanged.
neutron
+proton
+
-beta-minus
- "Neutron emission": A neutron leaves the nucleus; the element of the atom does not change, because it still has the same number of protons, but its mass decreases by one.
Below the green line (too many protons)
- "Beta-plus" decay: A proton emits a positron (a "positive electron") to form a neutron. This causes the neutron number to increase by one, and the proton number to decrease by one, so the atom changes element, but the mass number is unchanged.
+proton
neutron
+
+positron
- "Electron capture": A proton combines with an electron to form a neutron. This causes the neutron number to increase by one, and the proton number to decrease by one, so the atom changes element, but the mass number is unchanged.
+proton
+
-electron
neutron
- "Proton emission": A proton is ejected from the nucleus, causing the mass number and proton number to go down by one, which also changes the atom's element. Sometimes when an atom has a particularly large number of protons for its mass, it can lose two at once.
Isotopes with higher masses
As the mass of isotopes increases, especially beyond a mass number of about 210, their nuclei are no longer stable, often having both too many neutrons and protons, so they will often decay in a way that reduces the numbers of both.
- "Alpha decay": The nucleus emits an alpha particle made of two protons and two neutrons. This causes the neutron number to decrease by two and the proton number to decrease by two; this changes which element the atom is and decreases the mass number by 4.
218
84
Po
214
82
Pb
+
4
2
He
At very heavy masses, above about 255, many isotopes become so unstable that they decay by splitting apart in a process called "spontaneous fission". During spontaneous fission, one large unstable nucleus breaks into two smaller more stable nuclei, often releasing neutrons in the process. For any given fissile isotope, there is a range of possible fission reactions, producing a range of different daughter atoms.
Most nuclear decays also release "gamma radiation" after the first decay event. During gamma decay, the nucleus rearranges itself to a more stable form which releases a gamma ray photon, but does not change the mass number or proton number because no subatomic particles are lost.
You will also see that some isotopes have dual-coloured squares, meaning they undergo more complex decays, such as 'beta-minus then alpha' in which two decays happen simultaneously as part of the same event.
Try this in the simulation:
- On the 'Chart' screen, select any isotope on the graph by clicking on it. The colour of the square represents the decay mode (or stability) of the isotope - check the key for details.
- Toggling the 'Show gamma decays' option will add a white circle to any decays which also lead to gamma emission.
- Toggling the 'Decay mode' toggle to 'Complex' will show which elements undergo more complex decays than the straightforward alpha, beta, gamma, neutron model.