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This Buzzle article has a list of radioactive elements that abound in nature, arranged in the order of increasing atomic number, along with their decay modes.

Let us understand the phenomenon of radioactivity. Radioactivity arrived on the scene of world physics in the 19th century, just when people thought they knew everything in physics. With its discovery in 1896, radioactivity opened up a Pandora's box of questions and revealed a new world, waiting to be explored in the microcosm of the atomic nucleus.

What is Radioactivity?
Radioactivity is a very interesting phenomenon in nature. Classical Electromagnetism cannot explain radioactivity. It's a spontaneous and random phenomenon whereby nuclei of certain chemical elements like Uranium, radiate gamma rays (high frequency electromagnetic radiation), beta particles (electrons or positrons) and alpha particles (Helium Nuclei).

By the emission of these particles and radiation, the unstable nucleus gets converted into a stabler nucleus. This is called radioactive decay.

The Term 'Radioactive' - A Misnomer
A radioactive element is a fundamental element whose atomic nuclei demonstrates the phenomenon of radioactivity. The name 'radioactive' may suggest to you that radioactive elements radiate radio waves, but unfortunately that is not so! The name 'radioactivity' is a misnomer because these elements have nothing to do with radio waves! The reason is that energy and frequency of a gamma ray which is emitted by a radioactive element, is far beyond that of the radio band of electromagnetic spectrum! So, we are just stuck up with the name!

What Makes an Element Radioactive?
To understand radioactivity, we need to explore the structure of an atomic nucleus. Every nucleus contains neutrons as well as protons. Neutrons are neither positively charged, nor negatively charged, they are neutral particles. Protons are positively charged. As you might remember from high school physics, like charges repel each other while unlike charges attract each other. In the nucleus, protons and neutrons are cramped together in a really very small space.

The protons in the nucleus, all being positively charged, repel each other! So if all the protons repel each other, how does the nucleus stay glued together and remain stable? It is because of the 'Nuclear Force'.

This force is more stronger than the electromagnetic force, but the range of this force is only limited to size of the nucleus, unlike electromagnetic force whose range is infinite. This nuclear force acts between the protons and neutrons, irrespective of the charge and it's always strongly attractive. However, it has limitations of range. So, in the nucleus, there is a constant tussle between the repelling electromagnetic coulomb force of protons and the attractive strong nuclear force.

In a nucleus like Uranium, which has almost 92 protons, coulomb repulsive force becomes too much for the nuclear force to contain. Subsequently, the nucleus is very unstable and radioactive decay occurs and Uranium decays into a more stable element. Such an unstable nucleus like Uranium, when gently tapped by a neutron, splits up into two other nuclei through nuclear fission, releasing tremendous amount of energy in the process! This is the principle on which nuclear energy and nuclear weapons are based.

The radioactive elements listed below shows all the decay modes of Uranium. A full explanation of radioactivity can only be given, if we plunge deep into quantum physics and elementary particle physics.

Types of Radioactive Decay
This decay may occur in any of the following three ways:
  • Alpha Decay: Nucleus emits a helium nucleus (called an Alpha Particle) and gets converted to another nucleus with atomic number lesser by 2 and atomic weight lesser by 4.
  • Beta Decay: Beta decay could be of two types; either through emission of an electron or positron (the antiparticle of electron). Electron emission causes an increase in the atomic number by 1, while positron emission causes a decrease in the atomic number by 1. In some cases, double beta decay may occur, involving the emission of two beta particles.
  • Gamma Decay: Gamma decay just changes the energy level of the nucleus.
  • Electron Capture: One of the rarest decay modes is electron capture. In this phenomenon, an electron is captured or absorbed by a proton rich nucleus. This leads to the conversion of a proton into a neutron in the nucleus, along with release of an electron neutrino. This leads to a decrease in atomic number (transmuting the element in the process), while leaving the atomic mass number unchanged.
A radioactive element may have more than one decay mode.

Radioactive Isotopes
When two nuclei have the same atomic number, but different atomic weight or mass numbers, then they are said to be isotopes. Isotopes have the same chemical properties but different physical properties. For example, carbon has two isotopes, 6C14 and 6C12. Both have the same atomic number, but different number of neutrons. The one with the two extra neutrons is radioactive and undergoes radioactive decay. The radioactive isotope of carbon was used to develop carbon dating tool, which has made the dating of various relics possible.

Half-Life of a Radioactive Element
Half-life is the amount of time required, for half quantity of radioactive element to decay. For example C14has a half life of 5730 years. That is, if you take 1 gm of C14, then half of it will have been decayed in 5730 years. In the list presented below, half-lives of all the radioactive elements are presented.

Radioactive Elements List
Here is a detailed and comprehensive list of natural radioactive elements along with their atomic and mass numbers, decay modes and half-lives. Here 'Beta Decay (β-)' denotes electron emission while Beta Decay (β+) denotes positron emission.

Radioactive Element Atomic Number Atomic Mass Number Decay Type Half-Life
Hydrogen (H) 1 3 Beta Decay (β-) 12.32 years
Beryllium (Be) 4 7 Electron Capture (ε), Gamma Decay) 53.12 Days
Beryllium (Be) 4 8 Alpha 7 x 10-17 sec
Beryllium (Be) 4 10 Beta Decay (β-) 1,360,000 years
Carbon (C) 6 14 Beta Decay (β-) 5,730 years
Calcium (Ca) 20 41 Electron Capture (ε) 103,000 years
Calcium(Ca) 20 46 Double Beta Decay (β-β-) > 2.8 x 1015 years
Calcium(Ca) 20 48 Double Beta Decay (β-β-) > 4 x 1019
Iron (Fe) 26 54 Double Electron Capture (ε) > 3.1 x 1022 years
Iron (Fe) (Synthetic) 26 55 Electron Capture (ε) 2.73 years
Iron (Fe) (Synthetic) 26 59 Beta Decay (β-) 44.503 days
Iron (Fe) (Synthetic) 26 60 Beta Decay (β-) 2,600,000 years
Cobalt (Co) (Synthetic) 27 56 Electron Capture (ε) 77.27 days
Cobalt (Co) (Synthetic) 27 57 Electron Capture (ε) 271.79 days
Cobalt (Co) (Synthetic) 27 58 Electron Capture (ε) 70.86 days
Cobalt (Co) (Synthetic) 27 60 Beta Decay (β-), Double Gamma 5.2714 years
Nickel (Ni) 28 59 Electron Capture (ε) 76,000 years
Nickel (Ni) (Synthetic) 28 63 Beta Decay (β-) 100.1 years
Zinc (Zn) (Synthetic) 30 65 Electron Capture (ε), Gamma 243.8 days
Zinc (Zn) (Synthetic) 30 72 Beta Decay (β-) 46.5 hours
Selenium (Se) 34 79 Beta Decay (β-) 3.27 x 105 years
Selenium (Se) 34 82 Double Beta Decay (β- β-) 1.08 x 1020 years
Krypton (Kr) 36 85 Beta Decay (β-) 10.756 years
Rubidium (Rb) 37 87 Beta Decay (β-) 4.88 x 1010 years
Strontium (Sr) 38 89 Electron Capture (ε), Beta Decay (β-) 50.52 days
Strontium (Sr) 38 90 Beta Decay (β-) 28.9 years
Yttrium (Y) 39 90 Beta Decay (β-), Gamma 2.67 days
Yttrium (Y) 39 91 Beta Decay (β-), Gamma 58.5 days
Zirconium (Zr) 40 93 Beta Decay (β-) 1.53 x 106 years
Zirconium (Zr) 40 94 Double Beta Decay (β-) > 1.1 x 1017 years
Zirconium (Zr) 40 96 Double Beta Decay (β-) 2 x 1019 years
Niobium (Nb) (Metastable) 41 93 Beta Decay (β-),Gamma 16.13 years
Niobium (Nb) 41 95 Beta Decay (β-), Gamma 34.991 days
Molybdenum (Mo) 42 93 Electron Capture (ε) 4 x 103 years
Technetium (Tc) 43 99 Beta Decay (β-) 2.111 x 105 years
Ruthenium (Ru) 44 103 Beta Decay (β-), Gamma 39.26 days
Ruthenium(Ru) 44 106 Beta Decay (β-) 373.59 days
Palladium (Pd) 46 107 Beta Decay (β-), Gamma 6.5 million years
Silver (Ag) 47 111 Beta Decay (β-), Gamma 7.45 days
Tin (Sn) 50 126 Beta Decay (β-) 2.3 x 105 years
Antimony (Sb) 51 125 Beta Decay (β-) 2.7582 years
Tellurium (Te) 52 127 Beta
Decay (β-), Gamma
9.35 hours
Tellurium (Te) 52 129 Beta Decay (β-) 69.6 minutes
Iodine (I) 53 123 Electron Capture (ε), Gamma 13 hours
Iodine (I) 53 129 Beta Decay (β-) 15.7 million years
Iodine (I) 53 131 Beta Decay (β-), Gamma 8.02070 days
Xenon (Xe) 54 125 Electron Capture (ε) 16.9 hours
Xenon (Xe) 54 127 Electron Capture (ε) 36.345 days
Xenon (Xe) 54 133 Beta Decay (β-) 5.247 days
Cesium (Cs) 55 134 Electron Capture (ε), Beta Decay (β-) 2.0648 years
Cesium (Cs) 55 135 Beta Decay (β-) 2.3 million years
Cesium (Cs) 55 137 Beta Decay (β-), Gamma 30.17 years
Cerium (Ce) 58 144 Beta Decay (β-) 285 days
Promethium (Pm) 61 147 Beta Decay (β-), Gamma 2.6234 years
Europium (Eu) 63 154 Beta Decay (β-), Beta Decay (β+), Gamma 16 years
Europium (Eu) 63 155 Beta Decay (β-) 2 years
Iridium (Ir) (Synthetic) 77 188 Electron Capture (ε) 1.73 days
Iridium (Ir) (Synthetic) 77 189 Electron Capture (ε) 13.2 days
Iridium (Ir) (Synthetic) 77 190 Electron Capture (ε) 11.8 days
Iridium (Ir) (Synthetic) 77 192 Beta Decay (β-), Electron Capture (ε) 73.827 days
Iridium (Ir) (Synthetic, Metastable) 77 192 Gamma Decay 241 years
Iridium (Ir) (Synthetic) 77 193 Gamma Decay 10.5 days
Iridium (Ir) (Synthetic) 77 194 Beta Decay (β-) 19.3 hours
Iridium (Ir) (Synthetic, Metastable) 77 194 Gamma Decay 171 days
Lead (Pb) 82 210 Beta Decay (β-), Alpha 21 years
Bismuth (Bi) 83 210 Alpha 3 million years
Polonium (Po) 84 210 Alpha 138 days
Radon (Rn) 86 220 Alpha, Beta Decay (β+) 1 min
Radon (Rn) 86 222 Alpha 4 days
Radium (Ra) 88 224 Alpha 4 days
Radium (Ra) 88 225 Beta Decay (β-) 15 days
Radium (Ra) 88 226 Alpha 1,622 years
Thorium (Th) 90 228 Alpha 2 years
Thorium (Th) 90 229 Alpha 7,340 years
Thorium (Th) 90 230 Alpha 80,000 years
Thorium (Th) 90 232 Alpha 14 years
Thorium (Th) 90 234 Beta Decay (β-) 24 days
Proactinium (Pa) 91 234 Beta Decay (β-) 6.75 hours
Uranium (U) 92 233 Alpha 159,200 years
Uranium (U) 92 234 Alpha 245,500 years
Uranium (U) 92 235 Alpha 7.038 x 108 years
Uranium (U) 92 236 Alpha 2.342 x 107 years
Uranium (U) 92 238 Alpha 4.468 billion years
Neptunium (Np) (Synthetic) 93 237 Alpha 2.144 million years
Plutonium (Pu) 94 238 Alpha 87.74 years
Plutonium (Pu) 94 239 Alpha 2.41 x 104 years
Plutonium (Pu) 94 240 Alpha 6.5 x 103 years
Plutonium (Pu) 94 241 Beta Decay (β-) 14 years
Plutonium (Pu) 94 242 Alpha 3.73 x 105 years
Plutonium (Pu) 94 244 Alpha 8.08 x 107 years
Americium (Am) 95 241 Alpha 432.2 years
Americium (Am) (Metastable) 95 242 Alpha, Gamma 141 years
Americium (Am) 95 243 Alpha 7,370 years
Curium (Cm) 96 242 Alpha 160 days
Curium (Cm) 96 243 Alpha 29.1 years
Curium (Cm) 96 244 Alpha 18.1 years
Curium (Cm) 96 247 Alpha 15.6 million years

These radioactive isotopes have a lot of applications today, ranging from medicine to atomic energy. Since these radioactive elements are harmful, burning up radioactive waste or disposing it, is difficult. Every development in science and technology brings in new problems. Now, it's for us to decide, how we intend to use the power of technology placed in our hands.