Discovery of radioactivity by Henri Becquerel
The history of the nucleus of an atom induces another great discovery of alpha beta gamma rays.
In 1896 the French scientist Becquerel, while investigating the nature of the mysterious x -rays discovered by Rontgen a few months earlier, found that a photographic plate wrapped in thick black paper was affected by a sample of potassium – uranyl – sulfate placed over it.
In fact, any uranium compound would be effective the plate through covered by paper and kept away from light.
The obvious conclusion that some radiations emanating from the uranium compound could penetrate through the cover and attack the photographic plate.
This penetrating radiation had its source in uranium itself and Becquerel christened this amazing behavior of radioactivity.
Properties of these radiations were very similar to those of x -rays.
- These rays highly penetrating, they affected photographic plates, ionization of gases and would also induce the fluorescence in some substances.
- These rays not influenced by heat, light or chemical composition.
Who discovered radium and polonium?
Marie Curie found that the activity of mineral pitchblende was far greater than what was expected of its uranium content.
In 1898 Pierre and Marie Curie actually isolated two new elements Polonium and Radium, more radioactive compared to uranium, the heaviest atom known at the time.
In 1900, Debierne and Giesel discovered actinium which was also radioactive. Radioactive effects recognized early and this helps the isolation to a considerable extent.
It was immaterial how uranium and radium chemically combined. The same number of radium atoms will always have the same activity independent of the physical state or environmental conditions.
The phenomenon of radioactivity associated with atoms that are heavier than lead or bismuth.
The phenomenon of emission of radiation as a result of spontaneous disintegration in atomic nuclei was termed as radioactivity.
Alpha-beta gamma radiation properties
The radiations emitted by naturally radioactive elements were shown to split by an electric or magnetic field into three distinct parts, known alpha (α), beta (β) and gamma (ɣ) rays.
Properties of alpha particles
Alpha particle consists of a stream of positively charged particle which carries +2 charge and has mass number 4.
These particles are identical to the nuclei of the helium atom shown by Rutherford. Thus alpha particle is doubly charged helium ion (He⁺²) with atomic number 2 and mass number 4.
When an Alpha particle ejected from within the nucleus of an atom, the mother element loses two units of atomic number and four units of mass number.
Properties of beta particles
Beta particle made up of a stream of negatively charged particles. The beta particle is identical with electrons from a study of their behavior in electric and magnetic fields and from the study of their e/m values.
e/m = 1.77 × 108 coulombs/gm.
The ejection of a beta particle with mass number 0 and charge 1, results in the transformation of a neutron into a proton.
When a beta particle emitted from the nucleus, the daughter element nucleus has an atomic number one unit greater than that of the mother element nucleus.
Although beta particles and electrons are identical in their electrical nature and charge/mass ratio, there is a fundamental difference between them.
The ejection of an electron from an atom converts a neutral atom into a positively charged ion but leaves the nucleus undisturbed. The ejection of a beta particle changes the very composition of the nucleus and produces an atom of the next higher atomic number.
Properties of gamma rays
These consist of electromagnetic radiation of very short wavelength (λ ∼ 0.005 – 1 Å). These are high energy photons.
The emission of gamma rays accompanies all nuclear reactions. During all nuclear reactions there occurs a change in the energy of the nucleus due to the emission of alpha or beta particles.
The unstable, excited nucleus resulting from the emission of an alpha or beta particle gives off a photon and drops a lower and more stable energy state.
Gamma rays do not carry charge or mass, and hence emission of these rays cannot change the mass number or atomic of the mother nucleus.
What happens when a positron is emitted?
Works of the Curies and Rutherford yet another mode of nuclear transformation discovered. This involves the ejection of a positron from within the nucleus. This ejection made possible by the conversion of a proton into a neutron.
The ejection of positron lowers the atomic number one unit but leaves the mass number unchanged.
Neutrino emission in beta decay
Breaking down of a neutron into a proton and a beta particle creates a problem with the principle of conservation of angular momentum. Particles like neutron, proton, and electron have the spin angular momentum ± ½ (h/2π) each.
Angular momentum not balanced on the radioactive reaction. If the angular momentum of the proton and the electron are +½ (h/2π) they exceed the angular momentum of the neutron. If they oppose each other then the momentum becomes zero in violation of that of the neutron.
Pauli, therefore, postulated that along with the ejected beta particle another tiny neutral particle neutrino also ejected.
Neutrino has a spin angular momentum,
Sum of angular momentum of the particles ejected
Thus this is the same as that of the neutron.
The mass of the neutrino is around 0.00002 with respect to the oxygen scale. Ejection of an electron from within the nucleus should be represented as
Neutron → proton + electron + neutrino
Charge and mass of alpha-beta and gamma particles
Comparison of nuclear and chemical reactions
Nuclear reactions are different from chemical reactions in many respects:
Chemical reactions involve some loss, gain or overlap of outer orbital electrons of the reactant atoms. Such reactions cannot alter the composition of the nuclei so that the atomic number of the chemical reactions unchanged.
CH4 + H₂O → CO + 3H2
On the other hand cause of nuclear decay involves the emission of alpha, beta particles or positrons from inside the nucleus. Which leads to a change in the atomic number of the nucleus.
In some artificially induced radioactive decay reactions, neutrons absorbed by target nucleus producing isotopes. Nuclear reactions, therefore, leads either to the birth of another element or produce isotopes of the parent element.
The nuclear reactions are accompanied by energy changes which far exceed the energy changes in chemical reactions.
The energy evolved in the radioactive transformation of one gram of radium five hundred thousand times as large as the energy released when one gram of radium combine with chlorine to form RaCl₂.