### MODERN PHYSICS

MODERN PHYSICS

ATOMIC STRUCTURE

1. Atomic model:-

The description of the distribution of mass and positive and negative charges in an atom is called an atomic model.

2. Thomson’s Atomic model(1898):-

In this Atomic Model, the positive charge of an atom is thought to be distributed uniformly over the entire body of the atom with electrons embedded in it, similar to the seeds in a watermelon, so as to make the atom electrically neutral.

Drawbacks:-

This model failed to explain observed spectral lines emitted by various elements.

3. Lenard’s Atomic Model (1903):-

1. Lenard proposed his atomic model in 1903.

2. He proposed that atoms must have a lot of empty place in it.

3. The atom consists of electrons and similar tiny particles carrying positive charge.

4. Bohr’s model (1912):-

Neils Bohr proposed the following postulates regarding his atomic model.

1. The electrons in an atom revolve around the nucleus in circular orbits just like the planets revolve around the sun.

2. The electrons revolve only in certain permitted orbits called stationary orbits.

3. As long as electrons revolve in a particular orbit they do not radiate energy.
4. The angular momentum in a stationary orbit is equal to where n=1,2,3,………….
5. The electrons have a definite energy in a stationary orbit. Whenever an electron jumps from one stationary state to another, absorption or emission of energy takes place.
E2-E1=nhν (emission)
E1-E2=-nhν (absorption)

5. Why is Thomson’s model not approved?

Thomson’s atomic model failed to explain observed spectral lines emitted by various elements.

6. What is a stationary orbit

As long as the electron revolves in a particular orbit, its energy is constant and doesnot emit any electromagnetic radiation. Hence, such orbit is called Stationary orbit.

7. What is the important feature of Bohr’s Atomic Model that accounted for stability of an atom?

According to second postulate the electrons revolve only in certain permitted orbits called stationary orbits and do not radiate energy in such orbits. This explains the stability of atom.

8. State Bohr’s Quantum condition?

A Stationary orbit is one in which the angular momentum of electron is equal to an integral multiple of where h is Planck’s constant.

Angular Momentum of the electron= (where n=1,2,3……….)
This is called Bohr’s Quantum condition.

9. How is energy emitted or absorbed by an atom according to Bohr’s model? 10. What are the important features of Rutherford’s atomic model?

The important features of Rutherford’s atomic model are:-

1. The atom is extraordinarily empty because most of the alpha-particles went straight through the atom.

2. The atom consists of small positively charged nucleus and that the whole atomic mass is concentrated at its centre.

3. The nucleus consists of protons and electrons reside outside the nucleus.

4. The number of protons inside the nucleus is equal to the number of electrons outside the nucleus.

11. Explain the historical developments of atomic models?

Thomson suggested an atomic model in the year 1898. He stated that the positive charge of an atom is distributed uniformly over the entire body of an atom with electrons embedded in it. Thomson’s atomic model failed to explain the absorbed spectral lines emitted by various elements. In 1903 Lenard proposed that the atom must have a lot of empty space in it. Lenard’s atomic model could not explain the stability and mass of an atom. In 1911 Rutherford proposed an atomic model consisting of central massive positively charged nucleus made up of particles called protons. Outside the nucleus there are electrons which move around it at some separation. Rutherford’s model failed to explain the stability of an atom. In 1912, Bohr’s atomic model assumed stationary circular orbit of definite energy for electrons with nucleus at the centre. Bohr’s model explained the spectral lines emitted by different elements.

12. Drawback’s or Defects of Rutherford’s Atomic model:-

According to electromagnetic theory, the revolving electrons should lose energy continuosly. If the revolving electrons lose energy continuosly they come closer and closer to the nucleus and ultimately fall into the nucleus. This will lead to the collapse of the atom. So, Rutherford’s model cannot account for the stability of the atom.

ATOMIC NUMBER, ATOMIC MASS AND MASS DEFECT

1. What do you understand by nuclear atomic model

In nuclear atomic model, protons and neutrons exist inside the nucleus and electrons exist outside the nucleus.

2. Nucleons:-

Protons and Neutrons are collectively known as Nucleons.

3. Atomic Number (Z):-

The number of protons or electrons in an atom is defined as Atomic Number.

4. Atomic Mass Unit (a.m.u):-

The a.m.u is defined as of the mass of a neutral carbon (C) atom.

5. Atomic Mass:-

The total quantity of matter contained in an atom is called Atomic mass.

6. Atomic Mass Number or Mass Number:-

The total number of protons (Z) and Neutrons (N) in an atom is defined as its ‘atomic mass number’ or ‘Mass Number’.

7. Mass Defect:-

It is difference between the sum of the individual masses of constituents in a nucleus and the mass of the nucleus itself.

Explanation:-

Consider Helium atom. The Helium nucleus contains two protons and two neutrons.
Mass of one Proton=1.0078 amu.
Mass of one Neutron=1.0087 amu.
Mass of two Protons=1.0078×2
=2.0156 amu.
Mass of two Neutrons=1.0087×2
=2.0174 amu.
Total mass=2.0156+2.0174 amu.
=4.0330 amu.
But actual mass is 4.0026 amu. Mass defect=4.0330-4.0026 amu.
Mass defect=0.0304 amu.
Mass defect of Helium=0.0304 amu.

8. State the law of Einstein’s mass-energy equivalence?

Mass and energy are mutually convertible. The principle of mass-energy equivalence is given by
E=Δmc2.

9. Einstein’s Mass-Energy equivalence:-

According to Einstein, mass and energy are not independent. Any body which changes its energy content ‘E’ undergoes a change in its mass (m) so that the ratio is always a constant and equal to “C”. Mass and energy must be expressed in the same units. The relation between mass and energy is given by
E=ΔmC2.
Where,
E=Energy released or absorbed.
m=mass destroyed or created.
C=velocity of light.

10. What is Atomic Mass Unit?
Atomic Mass unit is a measure of atomic mass and it is defined as of the mass of a neutral carbon atom.

11. What is an electron volt?

Electron volt is the unit of energy. 1 eV is the kinetic energy of an electron which has been accelerated through a potential difference of 1 volt.

Note:-

Binding energy=mass defect x 931.5 MeV.
1 amu=931.5 MeV.
1 MeV=1.6×10-12 J.

12. How do you account for mass defect of an atom?

Every stable nucleus has a mass which is less than the total mass of it’s constituents and this difference is called Mass defect. The Mass defect accounts for the binding of the nucleons in the nucleus.

13. How is the stability of a nucleus measured?

The magnitude of the mass defect is the measure of the stability of a nucleus.

14. Binding Energy:-

The energy liberated when the nucleons are bound together in a nucleus is called Binding Energy
or
It is energy required to break the nucleus into its constituent protons and neutrons.

1. Natural radio activity:-

The self-disintegrating, spontaneous activity in the form of emission of radiations by some unstable nuclei with atomic number greater than 83 is called Natural radio activity.

2. Classification of Radiations from a radio active substance or analysis of radio active radiations by Rutherford. 1. A small quantity of a radio active material is placed in a thick block of lead.

2. Radio active rays come out from the cavity.

3. When electric field is applied, the radio active beam splits into three sections.

4. One section of radiations which is deviated by electric field towards the negative plate slightly is called α (Alpha) rays. These rays have positive charge.

5. The radiations which are deviated by electric field towards the positive plate to a larger extent are called β (Beta) rays. These rays have negative charge.

6. The radiations which are undeviated by electric field are γ (gamma) rays. They have no charge.

3. Properties of α rays:-

1. Alpha rays are nothing but Helium nuclei having two units of positive charge and four units of Mass.

2. The speed of α particles in air is of the order of 107 m/sec.

3. In an electric field, they are deflected towards negative plate slightly.

4. They have greater ionizing power than β and γ rays.

5. They have less penetrating power when compared to β and γ rays.

6. They cause fluorescence on substances like zinc sulphide.

4. Properties of β rays:-

1. β rays are fast moving electrons having unit negative charge.

2. The speed of β-particles is of the order of 108 m/s.

3. In an electric field they deflect towards positive plate to a larger extent.

4. They have less ionizing power than α rays.

5. The penetrating power of β-particles is more than that of α-particles.

6. They affect the photographic plates.

7. They cause fluorescence on substances like Zinc sulphide.

8. They cause radiation damage.

5. Properties of γ-rays:-

1. γ-rays are electro magnetic radiations.

2. γ-rays have no charge and mass.

3. γ-rays travel with the velocity of light.

4. γ-rays are not deflected by electric and magnetic fields.

5. They have less ionizing power than α and β rays.

6. They have very high penetrating power than α and β rays.

7. They affect the photographic plates.

8. They cause fluorescence on substances like Zinc sulphide.

9. γ-rays are used to kill cancer cells.

6. Differences between α,β and γ rays.
or
Compare the properties of α,β and γ rays.

 Properties α-rays β-rays γ-rays 1.  Nature Helium nuclei. Fast moving electrons Electro-magnetic radiations. 2.  Charge Two units of positive charge Unit negative charge No charge. 3.  Mass Four units of mass Negligible mass No mass. 4.  Velocity 107 m/s 108 m/s (3×108 m/s) equal to the velocity of light. 5.  Deflection in electric field Deflect towards negative plate slightly. Deflect towards positive plate to a large extent. No deflection. 6.  Ionizing power Very high Less than α-rays Very low 7. Penetrating power Very low Greater than α-rays Very high. 8.  Biological effect Harmful Harmful Highly dangerous.

7. Radio Active Transformation:-

The disintegration of an unstable nucleus resulting in another nucleus is called “Radio active Transformation”.

8. α-decay:-

If a radioactive transformation involves emission of α-particles, it is called α-decay. The ejection of a α-particle reduces the mass of the atom by four units and its atomic number by two units.

When α-particle is emitted, the position of the new element is two places to the left in the periodic table.

9. β-decay:-

If a radioactive transformation involves emission of β-particles , it is called β-decay. The ejection of β-particle does not change the mass number of the atom but increases its atomic number by one unit.

When β-particle is emitted, the position of the daughter element in the periodic table is one place to the right.

10. γ-decay:-

The emission of γ-rays does not alter either the atomic number or the Mass number of an atom. However, γ-decay results in the change in the energy state of the nucleus.

The sequential decay of a radioactive nucleus results in the production of a series of nuclei or elements. This series of elements, thus formed is known as ‘radioactive-decay-series’.
There are four radioactive series. They are

1. The Uranium .

2. The Thorium .

3. The Actino Uranium or Actinium Series.

4. The Neptunium .

1. The Uranium Series (4n+2):-

It starts with radioactive element uranium and ends with stable element lead.
The uranium series is also called ‘4n+2’ series in which the mass number of nuclei is divisible by 4 with remainder 2.

2. Thorium Series (4n):-

It begins with Thorium and ends with the stable element lead. This is also called ‘4n’ series in which the mass number of any nucleus is divisible by 4 with remainder 0

3. Actinium Series (4n+3):-

It Starts with Actino Uranium and ends with stable element lead. It is also called ‘4n+3’ series in which the mass number of any nuclei is divisible by 4 with remainder 3.

4. Neptunium Series (4n+1):-

It is also called 4n+1 series. This series ends with Bismuth. It is an artificial radioactive series.

12. Law of Radioactive Disintegration:-

In any Radioactive substance the number of atoms disintegrating per second is directly proportional to the number of atoms present.

13. Half-Life Period:-

The Half-Life period (T) of a radioactive substance is defined as the time required for half of its atoms to disintegrate.

14. Isotopes:-

Atoms of the same element having same atomic number but different mass numbers are called Isotopes.

Examples:-

15. Isobars:-

Atoms of the different elements having same mass number but different atomic numbers are called isobars.

Examples:-

16. Isotones:-

Isotones are the atoms with their nuclei having same number of Neutrons but different atomic numbers and mass numbers.

Examples:-

17. What are α,β and γ radiations?

α-rays are nothing but helium nuclei having two units of positive charge.
β-rays are fast moving electrons having a unit negative charge.
γ-rays are electro magnetic radiations.

18. In Rutherford’s experiment on classification of radio active radiations the radio active material is placed in a cavity of lead block. Why?

In Rutherford’s experiment on classification of radioactive radiations, the
radioactive material is placed in the cavity of block of lead because the radiations in all directions will be absorbed by lead.

19. The emission of α-particles by a radioactive element reduces the atomic number of the element by 2 units and mass number by 4 units. Why?

α-particles are nothing but Helium nuclei. It consists of two protons and two neutrons. Its mass number is 4. So, when it ejects α-particle its mass is reduced by 4 units and atomic number by 2 units.

20. Why does atomic number of a radioactive element increases by 1 unit on ejection of a β-particle?

When a radioactive element ejects a β-particle its mass number increases by 1 unit because a neutron in its nucleus gets converted into proton and a β-particle.

21. When a radioactive element ejects a β-particle it is transformed into another element. Why?

When a radioactive element ejects as β-particle a unit positive charge increases its nucleus. Thus, its atomic number increases by one unit. So, it is transformed into another nucleus.