NEET Study Notes on Atom and Nuclei: Atom is the smallest unit of an element. The properties displayed by them are relative to those of an element. These are minute in size. Atoms can be further divided into protons, electrons, and neutrons. The positive charge and mass of the atom are concentrated in the center which is called Nucleus.

• More than 99.9% mass of an atom is concentrated in the nucleus. The size of the atom is of order 10¹⁰ while that of the nucleus is 10-15 m.
• Candidates can expect at least 1 question each from Atom and Nuclei in NEET 2022 Physical Syllabus.
• The sample questions provided in this article will allow you to solve practical questions and prepare for this unit for NEET 2022.

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NEET Study Notes on Atom and Nuclei: Important Topics

NEET Study Notes on Alpha Scattering Particle Experiment

Rutherford α particle experiment

• In this experiment, the beam of alpha particles was made to pass through a hole that is small in the lead screen. This beam then falls on a thin gold foil. When the beam passes through the gold foil, alpha particles scatter through different angles.
• On the other side of the gold foil, a zinc sulfide screen is placed, this makes the alpha particles scatter at angles that vary from 0 to 180 degrees. When alpha particles fall on this screen, a flash of light is produced.
• This experiment led to finding that the alpha particles passed through the gold foil, The atoms that scattered at large angles collided with the particles. Only a few particles turned back to the source.
• Most of the positive charge and almost the whole of the mass is concentrated in the nucleus, in the small center. Electrons that revolve around the nucleus are unable to deflect the alpha particles. This demonstrates that electrons are light.
• Rutherford’s experiment demonstrated that the nucleus is the central, positive charged core with a size of 10^–15 m, whereas the number of electrons that surround the nucleus in a way that atom is electrically neutral.

Boh’s Model

In 1913, Bohr gave his hydrogen atom model. The postulates of Boh’s models are

• Electron moves around the nucleus in circular orbits due to the influence of the coulomb force of attraction between electron and nucleus.

• Electron rotates around the nucleus in stationary circular orbits, angular momentum is.
• An electron that is in stationary orbits does not radiate energy, this makes it stable. Atoms radiate energy moves from one stationery to another.

Read more NEET Study Notes on Structure of Atom

NEET Study Notes on Nucleus

Composition and Size of Nucleus

• Mass number- Total number of nucleons (neutrons+protons present in the nucleus of an atom.

• Atomic Number- number of protons= number of electrons

Types of Nuclei

• Isotopes- Atoms of elements that have similar atomic numbers but different atomic mass numbers.
• Isobars- Atoms of different elements which have the same atomic mass number but different atomic numbers. Examples- ₇N¹⁴, ₁₈Ar⁴⁰, ₂₀Ca⁴⁰
• Isotones- nuclides with the same number of neutrons are called Isotones. Examples- ₂He₂⁴, ₂Be₅⁹, ₅Be₅¹⁰ etc.
• Isomers- Isotones that have the same mass number, as well as the same atomic number with different radioactive properties, are called isomers.
• Size of the Nucleus- 1.1 × 10^–15 m.

Also Read:

• NEET Study Notes for Kinematics

• NEET Study Notes for Thermodynamics

What is Binding Energy?

The energy with which nucleons are bound in the nucleus is called the Binding energy of the nucleus. The work required to be done to separate nucleons that are an infinite distance apart from the nucleus in a way that they don't interact with each other is used to determine binding energy.

Total Binding Energy=

m_p = mass of the proton

m_n = mass of the neutron

Z= atomic number

A= Atomic mass

Binding Energy per Nucleon

Binding Energy per Nucleon of a Nucleus is the average energy that is required to extract the nucleon from the nucleus. Binding Energy per nucleon :

 

Binding Energy per nucleon helps in measuring the nucleus stability. The more is the binding energy per nucleon, the more stable the nucleus is. For smaller nuclei, the binding energy per nucleon is small.

Check NEET Study Notes on Current Electricity

Nuclear Force

The force that acts between nucleons in the nucleus is called nuclear force. It is responsible for binding the nucleon.

Characteristics of Nuclear Force

• Nuclear force is a force that is effective only in the short range of 10^–15 m
• It is independent of charge. Nuclear Force can act between proton-proton, proton-neutron,neutron-neutron.
• Nuclear Force is not a central force.
• The nuclear force is 10³⁸ stronger than gravitational force and 10² stronger than electric force.

Radioactivity

Spontaneous disintegration of the heavy nucleus of an atom that occurs without external force is called Radioactivity. The three main types of Radioactive radiation are

• α-rays
• β-rays
• γ-rays (gamma radiations)

• The unstable nuclei achieve stability by this process. The process is not affected by chemical contribution or changes in the physical environment.
• When the nucleus undergoes alpha or beta decay, atomic and mass number changes and transform into a new element. When the nucleus emits gamma rays, the mass and the charge of the nucleus do not change.
• The energy spectrum in Beta particles is in continuity but in alpha and gamma rays it shows a line spectrum. This shows that the emission of beta particles is without any amount of kinetic energy.

Read NEET Study Notes Gravitation

Properties of Alpha Rays

• Alpha Rays are positively charged particles with a charge of 3.2 × 10^–19 coulomb.
• Alpha particles get deflected in electric as well as magnetic fields.
• Mass of alpha particles is 6.645 × 10^–27kg.
• Range of alpha particles in air depends on the radioactive substance.

Properties of Beta particles

• Beta particles may be positively or negatively charged with a charge of .
• Beta particles deflect in electric and magnetic fields.
• Velocity of beta particles varies from 0.01c to .99c, where c is the velocity of light.
• Beta particles have ionization and penetration power. The ionization power is less than alpha particles whereas the penetration power is more than alpha particles.

Properties of Gamma Rays

• Such Rays are electromagnetic waves such as X-Rays.
• Since they are not chargeless, they are not deflected in electric and magnetic fields.
• Velocity of gamma rays or gamma particles is equal to the velocity of light.
• Ionisation power of gamma rays is less in comparison to alpha and beta rays, however, the penetration power is more.

The difference in properties of Alpha, Beta and Gamma Rays

Nuclear Fission

Nuclear Fission is the disintegration of a heavy nucleus into two lighter nuclei. Most of the energy is released due to the kinetic energy of the fission segment.

Uncontrolled chain reactions- When the number of fission increases at a tremendous rate which leads to the creation of huge amount of energy in a limited amount of time.

Nuclear Reaction is used to keep the number of fissions constant. The main parts of a nuclear reactor are

• Fuel- Fissionable material for the purpose of fission. Examples are U²³³, U²³⁵, Pu²³⁹
• Moderators- These slow down the fast neutrons or slow down the nuclear reaction. Examples are heavy water, graphite.
• Control Rods- These help in the absorption of excess neutrons and controls the chain reaction.
• Coolant- Coolant helps in removing the heat that is produced in the core of the heat exchanger for electricity production. Eg- air, water, and carbon dioxide.

Read:

• NEET Study Notes on Electronic Devices

• NEET Study Notes on Work Energy and Power

Nuclear Fusion

The fusion of two or more light nuclei to form a heavy nucleus with a large amount of release of energy is called Nuclear Fusion.

• High temperature is required to overcome coloumbic forces that act between nuclei, for nuclear fusion to take place.
• This is the principle of Hydrogen Bomb
• Per nucleon of the product, the binding energy is greater than the reactants.
• Reactants in large quantity are required for nuclear fusion to take place.
• The energy released per nucleon is -6.75 MeV.

Difference between Nuclear Fusion and Nuclear Fission

NEET Sample MCQs on Atom and Nuclei

Question: Most of the alpha particles go straight through the foil when passed through it. Why does this happen?

1. High speed
2. Positive charge
3. Negative charge
4. Empty space present in atom

Answer: Empty Space present in Atom

Question: The nucleus of an atom was discovered by which atomic mode?

1. Dalton Atomic Model
2. Bohr Atomic model
3. Rutherford atomic model
4. None of the above

Answer: Rutherford Atomic Model

Question: If electron, alpha particle, proton, and neutron have the same kinetic energy, which particle has the shortest wavelength?

1. Electron
2. Proton
3. Alpha particle
4. Neutron

Answer: Alpha particle

Question: What is the alpha particle the same as?

1. Hydrogen nucleus
2. Proton
3. Positron
4. Helium Nucleus

Answer: Helium Nucleus

Question: What are the nuclei with the same number of neutrons but different mass numbers called?

1. Isotherms
2. Isotones
3. Isotopes
4. Isobars

Answer: Isotones

Question: What is based on the principle of Nuclear Fusion?

1. Hydrogen Bomb
2. Atomic Bomb
3. Both Atomic and Hydrogen Bomb
4. None of the above

Answer: Hydrogen Bomb

Question: On what principle is Atomb bomb based?

1. Radioactivity
2. Nuclear Fusion
3. Nuclear Fission
4. None of the above

Answer: Nuclear Fission

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