Types of Radioactive decay
** According to the theory put forward by Rutherford and Soddy (1903), radioactivity is a nuclear property.
** The nucleus of a radioactive atom is unstable. It undergoes decay or disintegration by spontaneous emission of an α– or β-particle.
** This results in the change of proton-neutron composition of the nucleus to form a more stable nucleus. The original nucleus is called the parent nucleus and the product is called the daughter nucleus.
** As evident from above, there are two chief types of decay:
** When a radioactive nucleus decays by the emission of an α-particle (α-emission) from the nucleus, the process is termed α-decay.
** An alpha particle has four units of atomic mass and two units of positive charge.
** If Z be the atomic number and M the atomic mass of the parent nucleus, the daughter nucleus will have:
Thus an α-emission reduces the atomic mass by 4 and atomic number by 2.
** For example, Radium decays by α-emission to form a new element Radon,
** When a radioactive nucleus decays by β-particle emission (β-emission), it is called β-decay.
** A free β-particle or electron does not exist as such in the nucleus. It is produced by the conversion of a neutron to a proton at the moment of emission.
** This results in the increase of one positive charge on the nucleus. The loss of a β-particle from the nucleus does not alter its atomic mass. For a parent nucleus with atomic mass M and atomic number Z, the daughter nucleus will have:
** Thus a β-emission increases the atomic number by 1 with no change in atomic mass.
** An example of β-decay is the conversion of lead-214 to bismuth-214,
** It is noteworthy that a β-emission results in the production of an isobar. Thus, 214Pb82 and 214Bi83 are isobaric as they have the same mass number 214 but different atomic numbers (82 and 83).
** One α-emission and two β-emissions yield an isotope. Let us consider the following series of changes.
** The parent element 218Po84 emits an α-particle and subsequently two β-particles, resulting in the formation of 214Po84which is an isotope of the parent. Both the parent and the end-product have the same atomic number 84 but different mass numbers (218 and 214).
Problem (1):How many α and β particles are emitted in passing down from 232Th90 to 208Pb82?
Let (a) be the number of α particles and (b) be the number of β particles emitted during the radioactive transformation. It can be represented as:
Comparing the mass numbers, we get
Comparing the atomic numbers, we get
Substituting the value of a, we get
Thus the number of α particles emitted = 6 and the number of β particles emitted = 4
Problem (2): 210Pb82is a β-emitter and 226Ra88is anα-emitter. What will be the atomic masses and atomic numbers of daughter elements of these radioactive elements ? Predict the position of daughter elements in the periodic table.
(a) 210Pb82 undergoes β-decay i.e.
Comparing the atomic masses, we have
and comparing the atomic numbers, we get
Thus the daughter element will have the same atomic mass 210 and its atomic number will be 83. It will occupy one position right to the parent element.
(b) 236Ra88undergoes α decay i.e.
Comparing the atomic masses, we get:
and comparing the atomic number, we get:
Thus the daughter element will have atomic mass 232 and its atomic number will be 86. It will occupy two positions to the left of the parent element.
The group displacement law
** The position number of an element in a Group of the Periodic Table corresponds to its atomic number.
** If the atomic number of a given element is changed, its Group also changes accordingly.
** We know that an α-emission decreases the atomic number of the parent element by 2 and a β emission increases the atomic number by 1. Thus : in an α-emission, the parent element will be displaced to a Group two places to the left and in a β-emission, it will be displaced to a Group one place to the right.
** This is called the Group Displacement Law. It was first stated by Fajans and Soddy (1913) and is often named after them as (Fajans-Soddy Group Displacement Law).
Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolour edition.