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We have read that the entire mass of the atom and positive charge is concentrated in the nucleus and nucleus is composed by the protons and neutrons. We also know how much protons and neutrons occur in the nucleus therefore, the expected mass of a nucleus can be determined by the calculation. The actual mass of nucleus can also be determined by mass spectrograph. It is found that the actual mass of any nucleus is always less than the expected mass obtained by its nucleons calculation. This difference of mass is called mass defect. Similarly,
Mass defect = Mass of nucleus obtained by the calculation — Actual mass of nucleus
or Δm = m_c – m_a
Where calculated mass is represented by m_c and actual mass is represented by m_a.
∴ Δm = [Mass of protons + mass of neutrons] – Actual mass of nucleus
or Δm = [Zm_p +(A – Z) m_n] – m …………… (1)
Where Z is atomic number, A’mass number, m_p mass of proton, m_n mass of neutron and m is the actual mass.

Based on controlled reaction.

1. γ-rays travels with the speed of light (3 Χ 10⁸m/s) 

2. Alpha particle 

3. Rare gas is Helium (He)

“The phenomenon of some substances to give out spontaneous emission of invisible radiation which can penetrate through some thickness is called RADIOACTIVITY” and these substances are called RADIOACTIVE SUBSTANCES. 

Name of RADIOACTIVE SUBSTANCES : URANIUM, RADIUM, THORIUM, POLONIUM and ACTINIUM.

Whether the mass of a fissionable material can sustain a chain reaction or not, depends on it multiplication factor. This in turn depends on the size of the material. The size of the fissionable material for which the multiplication factor k = 1 is called critical size and its mass is called critical mass. The chain reaction in this case remains steady or sustained.

(c) neutron number decreases

1. Particle of both are electrons and are NEGATIVELY CHARGED. 

2. Both are deflected by electric and magnetic field. 

3. Both cause FLUORESCENCE. 

4. Both produce X-RAYS when stopped by metal of high ATOMIC NUMBER and high melting point (m.p.) such as, TUNGSTEN.

(a) On absorbing one electron, an a — particle (₂He⁴ nucleus) will change into an helium ¡on which is singly ionised. 

(b) On absorbing two electrons, it will change into an helium atom. 

(a) 1. When an a-particle is emitted, its atomic number decreases by 2. 

2. When a beat particle is emitted, the atomic number increases by 1. 

(b) α-particles and beta particles when subjected to an electric field are deflected. This is so because both the α - particles and beta particles are charged. Electric field exerts a force on them which deflects them. However, γ rays are not deflected because they are not charged and do not experience any force. Hence they are not deflected.

Particle is β(beta) i.e. electron ⁰ _-1e.

They are earth metals and transition metals.

They are used to study the function of fertilizers for different plants. They have also been used for developing new species of a plant by causing genetic mutations. They are also used in treatment of cancer.

1. α rays have the least penetrating power. 

2. Radioisotopes are used to study the function of fertilizer for different plants. They have also been used for developing new species of a plant by causing genetic changes. 

3. Background radiations are present at all places even in the absence of any radioactive source. The radiations are present in the atmosphere even when there is no source nearby. The source of background radiations are : 

(a) cosmic radiation 

(b) rocks in the earth which contain traces of radioactive substances 

(c) naturally occurring radioisotopes. 

During the emission of a beta particle, the Mass number remains the same.

1. α-radiation being heaviest and slowest causes most biological damage. 

2. When a-radiation is emitted, the atomic number decreases by 2.

Equivalent energy (in MeV) of lu mass is 931.5 MeV.

Energy released = 0.205 x 931 MeV 

= 190.86 MeV

When ²³⁵ ₉₂U decays to ²³⁵ ₉₂U, the mass number decreases from 238 to 206 i.e. it decreases by 32. Since emission of beta particle (β) does not change the mass number and with the emission of one a (alpha) particle, mass number decreases 4, so total number of alpha particles emitted will be 32/4 = 8. 

In the decay of ²³⁵ ₉₂U to lead ²⁰⁶ ₈₂Pb,the atomic number has decreased by 10. But due to emission of 8 αparticles, the atomic mass would have decreased by 2 x 8 = 16. Thus there is an increase in atomic number by 6, hence 6 Beta (β) particles will be emitted. (Because in emission of one beta particle, atomic number increases by 1). Thus ²³⁵ ₉₂U decays to ²³⁵ ₉₂U with emission of 8- α particles and 6 β-particles.

Gamma ray is not deflected by electric and magnetic field.

Correct answer is (d) 107 K.

Correct answer is (b) I¹³¹

Correct answer is (b) ₂₆Fe⁵⁹

(c) disease in the thyroid gland

Comparison of Ionising power of α, β,γ Heavier particle has high ionising power. 

a → 10,000 times of α and 100 times of β 

P → 100 times of α 

∴ Maximum ionising power a, minimum ionising power γ. 

Comparison of Penetrating power of α, β and γ 

Light particle has maximum speed hence maximum penetrating power. 

α has very large penetrating power up to a few hundred meter in air. 

α has small penetrating power (being very heavy) 3 to 8 cm in air

 β- has large penerating power up to few meter in air α < β < γ. 

The phenomenon by which even light elements are made radioactive, by artificial or induced methods, is called ‘artificial radioactivity’ or ‘man-made radioactivity’. 

This kind of radioactivity was discovered by Irene Curie and F.Joliot in 1934. Artificial radioactivity is induced in certain lighter elements like boron, aluminium etc., by bombarding them with radiations such as ‘alpha particles’ emitted during the natural radioactivity of uranium. This also results in the emission of invisible radiations and elementary” particles. During such a disintegration, the nucleus which undergoes disintegration is called ‘parent nucleus’ and that which is produced after the disintegration is called a ‘daughter nucleus’. The panicle, which is used to induce the artificial disintegration is termed as projectile and the particle which is produced after the disintegration is termed as ejected panicle. When the projectile hits the parent nucleus, it is converted into an unstable nucleus, which in turn decays spontaneously emitting the daughter nucleus along with an ejected particle.

If you denote the parent and daughter nuclei as X and Y respectively, then the nuclear disintegration is represented as follows: 

X (P,E) Y. Here, P and E represent the projectile particle and ejected particle respectively.

₄Be⁹ + ₂He⁴ → ₆C^13*

₆C^13* → ₆C¹² + ₀n¹

In the above nuclear reaction ₆C^13*  is unstable and is radioactive. This reaction can be represented as ₄Be⁹ (α, n) ₄C¹²

₄Be⁹  + ₂He⁴ → ₆C¹² + ₀n¹

(b) (ii) and (iii) are correct

Number and nature of decays = 3α and 2β 

Atomic number of isotope is Z = 92 

Mass number of isotope is A = 238 

Resulting isotope: 

After the emission of 3α particles atomic number 

Z’ = Z = (3 × 2) 

= Z – 6 

= 92 – 6 

Z’ = 86 

After the emission of 2β particles atomic number

Z” = Z – 2 (-1) = 86 + 2 = 88 

Mass number after the emission of 3α particles 

A’ = A – 3α 

= 238 – 3(4) 

= 238 – 12 

= 226 

Mass number after the emission of 2β particles 

A” = A’ – 2(0) 

= 226

∴ Resulting isotope is ₈₈Ra²⁶⁶

The phenomenon of nuclear decay of certain elements with the emission of radiations like alpha, beta, and gamma rays is called 'radioactivity'.

It is defined as the quantity of a radioactive substance, which produces 10⁶ disintegrations in one second. 

1 Rd = 10⁶ disintegrations per second.

This phenomenon of spontaneous emission of radiation from certain elements on their own is called 'natural radioactivity'.

This phenomena of spontaneous emission of radiation from certain elements on their own is called 'Natural radioactivity'.

(a) If both the assertion and the reason are true and the reason is the correct explanation of the assertion.

(b) more than 83

Two precautions to be taken while handling radioactive substance : 

1. Nuclear must be kept in thick lead container with narrow mouth and handled with mechanical tongs, wearing lead lined glasses. 

2. The workers must wear special film badges which can absorb nuclear radiation and should under go compulsory medical check up from time to time.

1. Use : Radiations from cobalt – 60 are used to treat cancer. 

Harmful Effect : The persons who are exposed to radiations these harmful radiations can kill the living tissues and cause damage. 

2. Cosmic rays from high altitudes and internal radiations given out by radioactive element present in the earth’s crust.

1. Nucleus of an atom become radioactive if 

n/p ratio is more than 1.5 i.e., number of neutrons are much more than number of protons in the nucleus 

Electrostatic force is “more than nuclear force. 

2. No change (:. Any physical or chemical change do not alter the rate of disintegration of radioactive substance). 

3. Slow moving neutron.

(a) Radioactivity is a self spontaneous disintegration of a heavy nucleus into α, β, and γ radiations. 

(b) Two differences between nuclear energy and chemical energy are as below: 

Nuclear energy 

1. 1. Nuclear energy is liberated from the nucleus of the atom in which protons and neutrons take part. 

2. 2. Nuclear energy produced is tremendous as 1 kg mass gets converted into 9 x 1016 J of energy. 

Chemical energy 

1. Chemical energy is liberated because of electrons which combine or release. 

2. A very small amount of energy is released in chemical reactions. It can be absorbed or released. 

Chemical change 

1. Change in number of orbital electrons takes place. 

2. Requires energy of few eV for a chemical reaction to take place. 

3. Number of atoms of each kind is conserved in reactants and products

Nuclear change 

1. Change in number of nucleons takes place. 

2. Nuclear change require much higher energy of the order of 106 times as compared to chemical change. 

3. Atomic number and mass number is conserved.

(c) 3.7 × 10¹⁰ becquerel

(c) If the assertion is true, but the reason is false.

(a) If both the assertion and the reason are true and the reason is the correct explanation of the assertion.

Correct answer is (b) Critical