• Atoms can be stable or unstable, and the instability is caused by the neutrons in their nucleus.
  • Unstable atoms are called radioactive, and when they decay, they release energy in the form of radiation.
  • There are three types of radiation: alpha, beta, and gamma.
  • All elements above atomic number 83 are radioactive, including Uranium (92), Plutonium (94), and Thorium (90).
  • The nucleus of these elements has a tendency to decay and release a large amount of energy.

Outcomes of Radioactivity

Think of these as balls of different sizes moving very fast. Alpha balls are small balls, Beta are even smaller, gamma is a photon (not a ball in the strict sense) that has high energy.

Alpha: 2 protons and 2 Neutrons moving at high velocity: Similar to Helium nucleus

If alpha balls are ejected, they hit other atoms and bounce off. After some hits and bounces they slow down finally to be absorbed. When they knock different particles, they strip off the electrons of those atoms making them ions. When alpha rays finally slow down, they usually attract 2 electrons (either free electrons or ones that were weakly attached to atoms) and form a helium atom.

Beta: Very Fast Electrons

Beta rays are energetic electrons. They too bounce off atoms hitting the electrons and striping them. The only difference is being electron they are much more feeble/weaker than alpha. However, though they are much lighter than alphas, they move so fast that they have energy comparable to that of the slower alpha particles. When betas finally stop (after numerous collisions), they usually attach themselves to an atom.

Gamma: High Energy Radiation (Photons)

Gamma rays on the other hand are high-energy rays (1 MeV) meaning they are traveling very fast (speed of light). They typically carry a million times as much energy as a single packet of visible light. So when they hit an atom the kinetic energy of gamma ray is enough to break the nucleus of that atom. This breakup of nucleus is called secondary radioactivity as you can well imagine.


Neutrons are massive particles like protons, but with no electric charge. Neutron emission is very important in both nuclear reactors and bombs.

Fission Fragments

Fission fragments are a particularly dangerous kind of radiation that is emitted when a nucleus undergoes fission, or it splits into two or more pieces. Fission fragments are chunks containing large numbers of protons and neutrons, and they are themselves highly radioactive. This is where the danger comes from. These are the radioactive particles that make fallout from nuclear bombs so dangerous. Fission fragments also make up the most radioactive part of nuclear reactor waste. 


We have already seen how an unstable nucleus ‘decays’/’transforms’/’changes’ etc. What does it really mean?

The stability and instability of nucleus is a matter of time it takes to break/change/transform. It so happens that nucleus of all atoms decay in accordance with a strange rule called ‘half-life’ rule. Accordingly, if you have a certain amount of nucleus (say x), only half of them (x/2) change in a certain time period called half-life (say y years). Rest of the nucleus(x/2) as expected will change in another y years? Not so. Out of the rest of the nucleus (x/2) only half will change in another y years. (x/4). So x/4 amount of nucleus remain and half of it changes in another y years and so on.

Applications of Radioactivity

  • Radioisotope Thermo-electric Generator (RTG): A radioactive material is used which when decays produces heat. This heat is in turn used by a generator to produce electricity. Eg: New Horizon spacecraft which went to Pluto uses this kind of device. Voyager which crossed our solar system recently also used RTGs.
  • Medical Imaging: Radioactive isotopes are used in medical imaging techniques such as X-rays, CT scans, PET scans, and MRI scans.
  • Radiation Therapy: Radioactive isotopes are used to treat various types of cancer through radiation therapy.
  • Smoke Detectors: Smoke detectors use a small amount of radioactive material to detect smoke and trigger an alarm.
  • Industrial Radiography: Radioactive isotopes are used in industrial radiography to test the integrity of metal structures such as pipelines and oil rigs.
  • Carbon Dating: Radioactive isotopes are used in carbon dating to determine the age of ancient fossils and artifacts.
  • Nuclear Power: Radioactive isotopes are used to generate electricity in nuclear power plants.
  • Food Irradiation: Radioactive isotopes are used to sterilize and preserve food products, preventing spoilage and disease.
  • Geological Dating: Radioactive isotopes are used to determine the age of rocks and minerals in geology.
  • Sterilization: Radioactive isotopes are used to sterilize medical equipment, surgical instruments, and other devices to prevent the spread of infection.

Radioactive Dating

C-14 is produced in the atmosphere due to the interaction of cosmic rays with nitrogen-14. The addition of neutron from this C-14 is breathed in by plants, consumed by secondary consumers etc. So, we all have certain amount of C-14 in our bodies as do plants and all living beings. Each gram of radioactive C-14 in our bodies decay at the rate of 12 per minute. (Means it becomes 1/2,1/4/,1/8,1/16 etc. every 12th minute.) The decayed C-14 is further replaced with atmospheric C-14 as we eat. This rate is same as that of atmospheric C-14. So, when you measure decay rate of C14 in living things you find they decay at 12 every minute. C-14 dating can be used to know anything that lived at some point.

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