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.