Comprehensive WBBSE Class 10 Physical Science Notes Chapter 7 Atomic Nucleus can help students make connections between concepts.
Atomic Nucleus Class 10 WBBSE Notes
Atom and its constituents:
| Inventor | Sub-atomic particles |
| J.J. Thomson (1897) | Electron |
| E. Rutherford (1911) | Proton |
| J. Chadwick (1932) | Neutron |
| Anderson (1932) | Positron |
| Yukawa (1935) | π – meson |
| Name given by Fermy (1956) | Neutrino |
Nucleus : From Rutherford’s experiment on scattering of α-particles, it was established that the positive charge of an atom and almost whole of its mass must be concentrated in a small region (10-12 -10-13cm) & (while dimensions of atoms 10-8cm)
Atomic number: The number of protons in the nucleus is called the atomic number of atom.
Mass number: The sum of the number of protons and neutrons is called mass number.
Nucleons : The protons and neutrons are collectively known as nucleons.
Nuclear reactions : Nuclear reactions are the reactions in which nucleus of an atom undergoes a change.
Nuclear chemistry: The branch of chemistry which deals with the phenomenon of nucleus of atoms is known as nuclear chemistry.
Type of species :
(i) Isotopes: The atoms of an element having same atomic number but different mass number are called isotopes.
(ii) Isobars : The nuclides of different chemical elements having same mass number but different atomic numbers are called isobars.
(iii) Isotones: The nuclides of different chemical elements having the same number of neutrons but different atomic numbers are called isotones.
(iv) Nuclear isomers : The nuclear species having same atomic number and same mass number but different radioactive properties are called isomers or nuclear isomens.
Examples:
(v) Isodiaphers : Atoms having the same isotopic number or isotopic excess (number of neutrons-number of protons) are called isodiaphers.
Examples : \(\quad{ }_{92}^{238}\) U and \({ }_{90}^{234}\) Th
(vi) Isoster : Molecules or ions with same number of atoms and also the same number of electrons are said to form isosteric group or more simply isosters.
Examples : In N2 O and CO2;
number of electrons = 22. So, they are isosters.
Radioactive radiations :
Characteristics of alpha (α), beta (β) and gamma (γ) – rays.
Stability of Nucleus : The stability of the nucleus is decided by the following factors:
(a) Nuclear forces: The forces which held the nucleons together within the small nucleus are called nuclear forces.
These force exist among p-p, p-n and n-n.
Nuclear force has been explained by yukawa by the discovery of a new fundamental particle called π-meson
These may be denoted as π+, π– and π0
The mesons keep on exchanging among the nucleons very rapidly (about 1024 transfers per second) and hold the nucleons together.
All nuclei with 84 or more protons i.e. atomic number ≥ 84 are unstable.
(b) Mass defect : Δm, the difference between the experimental and calculated masses of the nucleus is called the mass defect.
(Experimental mass of nucleus) – (mass of proton + mass of neutron) = mass defect.
(c) Binding energy: Atomic nucleus is made of protons and neutrons closely in a small volume. Although there exist intensive repulsive forces between the component protons, the nucleus is not slit apart. This is so because the nucleons are bound to one another by very powerful forces. The energy that binds the nucleons together in the nucleus is called the nuclear binding energy.
The binding energy of a nucleus can be calculated from its mass defect by using Einstein’s equation,
Δe = Δ m × c2 or Binding energy = Δm × 931 MeV
By plotting the binding energy per nuclear against the mass number, we get the graph shown in figure below.
(d) Packing fraction : Packing fraction was proposed by Aston and defined as the difference of actual isotopic mass and the mass number.
Packing fraction =\(\frac{\text { (acutal isotopic mass-mass number) }}{\text { mass number }} 10^4\)
The value of packing fraction may be negative, positive or zero. A negative packing fraction indicates less stability of nucleus. In general, for lower packing fraction the binding energy will be greater and the nucleus will be more stable.
(e) Neutron-Proton ratio (N / P ratio) : The stability of a nucleus seems depend on the neutron to proton (n / p) in the nucleus.
(i) In lower elements (upto z=20 ), the stable nuclei have about equal number of protons and neutrons i.e. \(\frac{n}{p}\) = 1
(ii) For higher elements to be stable, there must be more neutrons than protons i.e. \(\frac{n}{p}\) >1.
(iii) The shaded portion in figure represents the region or belt of stability. The elements whose n / p ratios lie inside the belt are stable.
(iv) A nucleus whose n / p lies above or below the stability belt is radioactive or unstable on account of unfavourable n / p ratio. It emits α or β-particles so as to move into the stability range.
Know more :
(a) If \(\frac{n}{p}\) > 1 to 1.5 i.e. it places above the stability belt. Such nuclei emit- β particle in order to lower its \(\frac{n}{p}\) ratio
(b) If \(\frac{n}{p}\) < 1 to 1.5 i.e. places below its stability belt, such nuclei either emit positrons or undergo electron capture.
(c) The even odd nature of the number of protons and neutrons : The number of stable nuclides is maximum when both p and n are even number. However, the number of stable nuclides in which either the z(p) or n is odd is about one third of those where both are even.
The magic numbers : Magic numbers are the numbers 2, 8, 20,50, 82 and 126. Nuclides having magic number of either proton or neutrons or both are more stable. Nuclides with atomic numbers equal to magic numbers have more number of stable isotopes than their neighbours. Nuclides having even number of protons and neutrons are more stable than nucli containing odd number of these particles.
Radioactivity : It is a process in which nucleus of certain elements undergo spontaneous disintegration without excitation by any external means. This phenomenon was first discovered by Henry Becquerel (1866). However, the term radioactivity was proposed by Madam Curie.
Group Displacement Law (Fajans and Soddy in 1913) : We know that an α-emission decreases the atomic number of the parent by 2 and 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 β emission, it will be displaced to a group one place to the right’.
Disintegration Series : The whole series of elements starting with the parent radioactive element to the stable end product is called a radioactive disintegration series.
Radioactive Decay: If N be the number of undecay atoms of an isotope present in a sample of the isotope, at time t, then
\(\frac{-d N}{d t} \alpha N\)
or, \(\frac{-d N}{d t}=\lambda N\)
where, \(\frac{-d N}{d t}\) tmeans the rate of decrease in the number of radioactive atoms in the sample and λ is the proportionality constant. This is known as disintegration constant.
Disintegration constant : It may be defined as the proportion of atoms of an isotope decaying per second.
Various forms of equation for radioactive decay are :
(i) Nt = N0 e-λt
(ii) λ = \(\frac{2 \cdot 303}{t}\) log \(\frac{N_0}{N_t}\)
(N0= No. of atoms initially
Nt = No. of atoms after time t)
Half-Life period : Half-life period of a radioactive isotope is the time required for one-half of the isotope of decay.
t1/2 = \(\frac{0.693}{\lambda}\)
The value of λ can be found experimentally be finding the number of disintegrations per second with the help of a Geiger-Muller Counter.
The unit of half life period : time-1
Average life : The statistical average of the lives of all atoms present at any time is called the ‘average life’.
τ = 1.44 × t1/2
(τ = Average life
τ = \(\frac{1}{\lambda}\))
Activity of a Radioactive Substance :
- Higher is the activity of a substance, faster will be its disintegration and vice-versa.
- The greater the half-life of the substance, lesser is its activity and vice-versa.
- The activity of a radioactive sample is usually determined experimentally with the help of a Gieger Muller counter.
Radioactive Equilibrium : Radioactive change being an irreversible process shows equilibrium when a daughter element disintegrates at the same rate at which it is formed from parent element.
Unit of Radioactivity :
- Curie : If a radioactive substance disintegrates at the rate of 3.7 × 1010 disintegrations per second, its activity is said to be 1 curie.
- Rutherford : If a radioactive substance has 106 disintegrations per second, it is said to have an activity of one Rutherford.
- Becqurel (SI unit) : If a radioactive substance has 1 disintegration per second, it is said to have an activity of one Becquerel.
Measurement of Radioactivity : The instruments used are
- Geiger-Muller counter
- Wilson cloud
- Electroscope
Natural transmutation : It is a process in which elements such as radium undergoes transmutation on their own.
Artificial transmutation: It is the process in which a stable nuclei is changed into another by artificial methods i.e. by bombarding the target nuclide with projectiles such as α-particles, neutrons etc.
The first artificial transmutation was carried out by Rutherford (1919) by bombarding nitrogen with α-particles.
Artifical radioactivity : It is the phenomenon in which artificial transmutation of a stable nucleus leads to the formation of a radioactive nuclide.
Contributions of Artificial Transmutation : The following are the important contributions of artificial transmutation :
(i) Discovery of neutron
(ii) Artificial radioactivity
(iii) Nuclear fission
(iv) Nuclear fusion
Alchemy : The process of transforming one element into other is known as alchemy and the person involved in such experiments is called alchemist.
Nuclear reaction: It is a reaction in which the number of protons in the nucleus of an element changes to form a new element.
Difference between Nuclear and Chemical Reactions :
| Nuclear reactions | Chemical reactions |
| (i) Proceed by distribution of nuclear particles. | (i) Proceed by the rearrangement of extra nuclear electrons. |
| (ii) One element may be converted into another. | (ii) No new element can be produced. |
| (iii) Often accompanied by release or absorption of enormous amount of energy. | (iii) Accormpanied by release or absorption of relatively small amount of energy. |
| (iv) Rate of reaction is unaffected by external factors such as concentration, temperature, pressure and catalyst. | (iv) Rate of reaction is influenced by external factor. |
Types of Nuclear reactions-
A. On the basis of mechainism
(i) Projectile capture reaction : The bombarding particle is absorbed with or without the emission of γ radiations
Example:
(ii) Particle-particle reactions : Majority of nuclear reactions come under this category
B. On the basis of bombarding particle
Nuclear Fission : The process of artificial transmutation in which heavy nucleus is broken down into two lighter nuclei of nearly comparable masses with release of large amount of energy is termed as nuclear fission.
(i) The first nuclear fission to be discovered was that of \({ }_{92}^{225} \mathrm{U}\).
(ii) Over 200 different isotopes of 35 different elements have been found among the fission products of \({ }_{92}^{225} \mathrm{U}\). Most of them are radioactive.
(iii) During fission, there is always lose of mass which is converted into energy according to Einstein equation.
E = Δ mc2(Δ m = mass defect)
(iv) It has been found that at the moment, the energy available from 1 kg of uranium is equivalent to that available from 20,000 kg of coal
Application of Nuclear fission –
(i) Nuclear reactor: A nuclear fission reactor is a device that permits a controlled chain nuclear fission, control rods made of elements such as boron and cadmium, absorb additional neutrons and can therefore, show the chain reaction.
(ii) Atom bomb : It is based on uncontrolled chain reaction. In the atom bomb fissionable material ( 235U or 239Pu) is taken in parts in such a way that each is in sub-critical stage. At the time of explosion these pieces one driven together rapidly by using explosives like TNT lying behind each piece of fissonable material make one large piece of fissonable material. At this instant, the overcritical stage is achieved and a fast chain reaction is set up. This result is a violent explosion with the release of tremendous amount of energy.
Nuclear fusion : A nuclear reaction in which two lighter nuclei are fused together to form a havier nuclei is called nuclear fusion.
The process of fusion can take place at extremely high temperature only (>106 K). Such reactions are known as thermo-nuclear reactions.
Application of Nuclearfusion :
(i) Energy of sun : Every second, the sun loses 4.3 × 109 kg (4,20,000 tons) of mass by the fusion reactions. This mass is converted to energy.
(ii) Hydrogen bomb : Hydrogen bomb is based on the fusion of hydrogen nuclei into heavier ones by the thermonuclear reactions with release of enormous energy. In hydrogen bomb, a mixture of deuterium oxide (D2O) and tritium oxide (T2O) is enclosed in a space surrounding an atomic bomb. The temperature produced by the explosion of atomic bomb initiates the fusion reaction between \({ }_1^3 \mathrm{H}\) Hand \({ }_1^2 \mathrm{H}\) Hreleasing huge amount of energy.
Difference between Nuclear Fission and Nuclear Fusion :
| Nuclear fission | Nuclear fusion |
| (i) A bigger nucleus splits into smaller nuclei. | (i) Lighter nuclei fuse together to form the heavier nucleus. |
| (ii) It does not require high temperature. | (ii) Extremely high temperature is required for fusion to take place. |
| (iii) A chain reaction sets in. | (iii) It is not a chain reaction. |
| (iv) It can be controlled and energy released can be used for peaceful purposes. | (iv) It cannot be controlled and energy released cannot be used properly. |
Application of Redioactivity
