- Rutherford discovered nucleus in his alpha particles scattering experiment. According to his observations, the whole mass of an atom is concentrated at the centre in very small volume this small portion is termed as a nucleus. The electrons revolve around the nucleus.
- From x-ray technique, it is observed that the charge on nucleus is Ze and it is positive. Where e is the magnitude of charge on one electron. Thus it was considered that the nucleus consists of z number of positively charged particles. Each particle carries a charge of + e. These particles are termed as protons.
- In 1932 Chadwick proved the existence of neutral particles in the nucleus. These neutral particles are called as the nucleus. Mass of neutron and proton is almost same.
- The particles protons and neutrons present in the nucleus are collectively called as the nucleons.
- The number of protons present in the nucleus of an atom is called atomic number. It is denoted by letter ‘Z’.
- The number of neutrons present in the nucleus of an atom is known as neutron number. It is denoted by ‘N’
- The total number of protons and neutrons present in the nucleus of an atom of the element is called mass number. The mass number is denoted as ‘A’.
A = Z + N or N = A – Z
- The mass of an atom is measured in a unit called atomic mass unit (a.m.u.)
Characteristics of Protons:
- Protons are positively charged.
- They are located in the nucleus.
- mass of proton is 1.0078 a.m.u.(1.672 x 10 -27 Kg.). This mass of a proton is considered as unit mass ( 1 a.m.u. ).
- Mass of one proton is almost equal to the mass of one H atom.
- A proton carries a positive charge of 1.6 x 10-19 C. This charge carried by the proton is considered to be a unit positive charge.
- Proton is denoted by 1H1 or 1P1. I.e. it has unit positive charge and unit mass.
Characteristics of Neutrons:
- Neutrons have no charge i.e. they are electrically neutral.
- They are located inside the nucleus of an atom.
- Mass of neutron is of 1.008665 a.m.u (1.675 x 10-27 Kg ). For practical purpose, this mass is assumed as unit mass.
- Mass of neutron is nearly equal to that of a proton.
- Neutron is denoted as 0n1.
- The phenomenon of spontaneous and continuous and uncontrollable disintegration of an unstable nucleus accompanied by the emission of active radiations is called natural radioactivity.
- The substance which exhibits radioactivity is called radioactive substance. e.g. Uranium, thorium, radium etc.
Characteristics of Natural Radioactivity:
- These characteristics are also called as Rutherford-Soddy’s radioactive disintegration theory.
- Natural radioactivity is a purely nuclear phenomenon. The nucleus of a radioactive substance is unstable and such unstable nucleus undergoes spontaneous breakdown (disintegration). The process continues till a stable nucleus is obtained.
- As Natural radioactivity is the nuclear phenomenon it is unaffected by chemical combination. I.e. the element will exhibit radioactivity in free as well as the combined state.
- Natural radioactivity is a spontaneous process. It is independent of external factors like temperature, pressure and state of existence of substance or catalytic action. Hence the process of radioactive disintegration is uncontrollable using these factors.
- The nucleus of radioactive element emit alpha, beta particles and gamma radiations and gets converted into the nucleus of another element.
- The elements undergoing disintegration is called parent element and a new element formed is called a daughter element. Daughter element has different chemical and physical properties as compared with that of its parent element.
- During disintegration, besides emission of alpha and beta particles and gamma radiation, a large amount of energy is liberated in the form of gamma rays. When gamma rays are given out no new element is formed.
- The time taken by a radioactive substance to disintegrate half of its initial quantity is called as half-life period. Half-life period is a characteristic property of every radio element.
- When radioactive substance emits one alpha particle mass number of daughter element reduces by 4 units and atomic number reduces by 2 units. When a radioactive substance emits one beta particle, the atomic number of daughter element increases by one unit but the mass number remain unchanged.
- The rate of disintegration at any instant is directly proportional to the radioactive nuclei present at that instant.
- Thus the rate of disintegration depends on nature and the original amount of the radioactive substance.
Rutherford’s Experiment to Study Natural Radioactivity:
- Rutherford analysed radiations of radioactive substance. He reported three types of radiations depending upon the effect of the magnetic or electric field upon them. These are alpha and beta particles and gamma rays.
- The apparatus consists of an evacuated metal chamber with a photographic plate at the top. A small quantity of radioactive substance is placed in a hole of the lead block. A strong electric field is applied between the plates.
- When there is no electrical field the radioactive emissions move in a straight line but when the electrical field is applied the emission get split into three distinct points on the photographic plate.
- The rays which get deviated towards the negative plate are positively charged and are called as alpha rays. Deflection of alpha particles is slightly less.
- The rays which get deviated towards the positive plate are negatively charged and are called as beta rays. The deflection of beta particles is more.
- The rays which do not get deviated and move straight are non charged and are called as gamma rays.
- By applying strong uniform magnetic field at right angles to the diagram same effect can be observed.
Characteristics of α – rays:
- These are positively charged particles. So α-rays are called α – particles rather than α -rays.
- Actually, these particles are helium nuclei (2He4) having 4 unit mass and 2 unit of positive charge.
- They are deflected towards the negative plate of the electric or the magnetic field.
- They have greater ionising power.
- They have the least penetration power.
- They can affect a photographic plate.
- They travel in straight line.
- They have a velocity which is about 1/10th that of light.
- When radioactive substance emits one µ -particle, the mass number of daughter element reduces by 4 units and atomic number by 2 units.
Characteristics of β-rays :
- β – Rays are negatively charged particles. So they are called β – particles rather than β – rays.
- β – particles are nothing but high-velocity electrons (-1e0) having unit negative charge and negligible mass.
- These rays are deflected towards the +ve plate of the electric or the magnetic plate.
- They have less ionising power as compared with that of α- rays
- They have greater penetration power than that of α – rays
- They affect a photographic plate to the much higher extent than the α – particles.
- They do not travel in straight line.
- They have a greater velocity than that of the α- rays very close to that of light.
- When a radioactive substance emits one β -particle, the atomic number of daughter element increases by one unit but the mass number remain unchanged.
Characteristics of γ – rays.
- gamma rays are non-material
- They are electromagnetic radiations.
- They are chargeless, hence remain undeflected due to the electric or the magnetic field.
- They have very low ionising power.
- They have high penetration power.
- They have a very little effect on a photographic plate.
- They travel in straight line.
- They have a velocity equal to that of the light.
- When radioactive substance emits gamma rays there is no change in the atomic number and the mass number.
Soddy’s Group Displacement Laws:
- Whenever parent element emits an α- particle, the daughter element produced has the atomic number less by 2 units and the mass number less by 4 units, so daughter element occupies 2 positions to the left with respect to its parent element in the periodic table.
- Whenever parent element emits one β – particle, the daughter elements produced has the atomic number greater by 1 unit but the mass number remains same. So daughter element occupies one position to the right with respect to its parent element in the periodic table.
- Whenever parent element emits one γ – ray, the daughter element produced has the same atomic number and the same atomic mass. So daughter element occupies same as its parent element in the periodic table.