Structure of Atom Chapter 2 Class 11 Chemistry Notes

Introduction

The Structure of Atom chapter 2 covers detailed information on subatomic particles, atomic models, and theories of different scientists, as well as the Quantum mechanical model. The concepts of atomic mass, atomic number, isotopes, and isobars have been included here. The Structure of Atom Class 11 Notes provide detailed information about the discovery, nature, and behavior of atoms. 

Definition of Atomic structure

Atomic structure refers to the constitution of an atom and the arrangement of its subatomic particles. The nucleus contains Protons and Neutrons, and the  Electrons orbit around the nucleus. 

Properties of Cathode Ray

1. Cathode rays travel in a straight line. 
2. Cathode rays travel from the cathode to the anode.
3. Cathode rays contain negatively charged particles. 
4. Cathode rays transfer kinetic energy to the objects with which they strike. 

Properties of Anode Ray

1. Anode rays contain positively charged particles. 
2. Anode rays travel in a straight line. 
3. Anode rays are deflected by the magnetic and electric fields. 
4. They cause the ionization of gases. 

Subatomic particles

We learn about the characteristic features of the particles of an atom in the Structure of an Atom chapter 2, Class 11 Chemistry.

Electron –

• Charge of proton is -1.602 x  10-19 coulombs.. 
• Mass of proton is 9.1 X 10-31
• It has negative charges. 

Proton – 

• Protons possess a positive charge. 
• Charge of proton is +1.602 x  10-19  coulombs.. 
• Mass of proton is 1.672 × 10-24
• Protons are 1800 times heavier than electrons. 

Neutron

• Mass of proton is 1.674 x 10-24
• It is neutral particles that have no charges. 

Atomic Models

Dalton, Thomson, Rutherford, and Niels Bohr are noble scientists who researched the structure of the atom by using some atomic models. Their opinion about the structure of the atom has been discussed in the Structure of Atom Chapter 2. 

Thomson Atomic Theory

1. J.J. Thomson described an atom as a sphere having a radius of 10 -10 m in which negatively charged electrons are embedded. 
2. In this theory, electrons are described as embedded in an atom just like a plum in a pudding. So, this theory is also called the Plumber Pudding theory.
3. Thomson said that positively charged particles are uniformly distributed with the arrangement of the electrons. That makes an atom electrostatically stable.

The mass of the atom is evenly spread throughout the atom.

Limitations of Thomson’s Theory 

• Thomson describes the neutrality of the atom clearly, but it lacks experimental evidence. 
• There is no proper explanation of the scattering experiment of Rutherford. 

Rutherford Atomic Theory

1. Most of the mass and the positive charge are concentrated in a small, specific area in an atom. This area is called the nucleus of an atom.
2. Electrons move around the nucleus in a circular path at a high speed. The path is called an orbit.
3. Electrons and the nucleus stay together because of the electrostatic forces of attraction.

Atomic number – The number of protons present in an atom is called the Atomic number.

Mass number – The total number of protons and neutrons is considered the Mass number of an atom.

Limitations of the Rutherford Atomic Model-

1. Rutherford’s theory can not describe the distribution of electrons that move around the nucleus. 
2. According to Rutherford, an electron moving at a high speed in its orbit should possess acceleration. As per Maxwell’s theory of electromagnetic, a particle having acceleration must emit electromagnetic radiation. So, electrons should spiral into the nucleus, but it doesn’t happen. Thus, this theory is unable to explain the stability of the atom.

Atomic structure of an Isotope

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

Name	Symbol	Mass Number	Number of Proton	Number of Neutron
Protium or hydrogen	1H1	1	1	0
Deuterium or Heavy hydrogen	1H2	2	1	1
Tritium	1H3	3	1	2

 Bohr’s Atomic Theory

1. The specific circular paths in which electrons move around the nucleus are called orbits. 
2. The orbits possess definite energy, which is called the energy level or energy cells.

Achievement – 

• This theory describes the line spectrum of hydrogen.
• It explains clearly the stability of the atom.

Limitations of Bohr’s Atomic Theory

• The theory could not explain the atomic spectra of multielectron atoms.
• This theory does not explain the fine structure of the spectral line.
• Bohr’s Theory does not cover the explanation of the capacity of an atom to create a molecule through a chemical bond. 

Dual Nature of Matter

De Broglie demonstrated that matter exhibits dual behavior, possessing both the properties of a wave and a particle. 

He proposed the relation between momentum and wavelength—

   λ = h/mv = h/P

m – Mass of particle

V- Velocity of particle

p- momentum of the particle

This relationship has been proved by experiment.

Electromagnetic Wave Theory 

The fundamental concept as well as equations regarding Electromagnetic waves, have been included in the Structure of Atom chapter 2. This theory was coined by Maxwell in 1864. 

1. Energy produced from a source in the form of radiation is called radiant energy. 
2. Radiant energy possesses electric and magnetic fields. Those fields are perpendicular to the direction of the radiation. 
3. It has the characteristics of a wave, and it doesn’t require any material medium to travel.

Photoelectric effect

When a ray of light having a certain frequency strikes a metal surface emits electrons; this phenomenon is called the photoelectric effect. Black Body radiation – An ideal body that absorbs as well as emits any frequency is called a black body. The radiation emitted by it is called black body radiation. The distribution of the frequency of the radiation emitted from the black body depends on its temperature.

Dual behavior of Electromagnetic radiation

Electromagnetic radiation possesses a dual nature, similar to the nature of both a wave and a particle. When it interacts with a matter, it acts like a particle and at the time it propagates, it shows the wave-like properties. 

Electromagnetic spectrum

When the electromagnetic radiations are arranged in order of increasing wavelength and decreasing frequency. The complete spectrum formed is called the electromagnetic spectrum.

Spectrum

When the ray of white light passes through a prism, the wave with shorter wavelength bends more than the one with a longer wavelength. White light consists of the waves of various wavelengths, and it spreads into a series of colored bands called the spectrum. The types of spectrum are one of the most important topics of the Structure of Atom chapter 2 and they are as follows:

• Continuous Spectrum–

The continuous spectrum has a full range of wavelengths in a specific range. When rays of white light pass through a prism, the light is scattered into seven colors. This spectrum is called a continuous spectrum. 

• Emission Spectrum—

When specific rays of us hi hu u from coming from a source pass through a prism and then fall on a photographic plate, it is called Emission.

• Absorption Spectrum—

When white light passes through vapour, it strikes a prism and dark lines appear. The dark line indicates that the radiation is being absorbed by the substance from the light. This spectrum is called Absorption.

• Line Spectrum—

Vapour of a volatile substance, when it falls on the flame of a Bunsen burner and analysed through a spectroscope, some colored lines emerge on a photographic plate that differ depending on the substance

• Pauli Exclusion Principle—

According to the principle, two electrons of an atom can not have the same set of quantum numbers. 

Quantum Mechanical Model of an Atom

• Quantum mechanics – It is a kind of theoretical science that involves the study of the motion of microscopic objects. The objects possess wave-like as well as particle-like properties.
• Features of the Quantum Mechanical Model of the Atom

1. The energy of electrons is quantized.
2. The momentum and position of electrons can not be determined 
3. Electrons stay in an atomic orbital and possess specific energy. More than two electrons can stay in the same orbital.

Conclusion

The Structure of Atom chapter 2 provides a complete understanding of the composition of an atom, the concept of spectrum, isotope, and Quantum theory. The chapter Structure of an atom plays a significant role in achieving a good score in the CBSE board exam and JEE. A proper understanding and the topic is essential to meet your desired goal. 

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