NCERT Class 11 Chemistry Chapter 2 Notes Structure of Atom - Download PDF

Everything in the universe is made up of atoms, from the tiniest drop of water to the vast universe. But can you imagine what an atom looks like? Can you see beyond that teeny tiny structure? Well, some scientists did. This chapter Structure of Atom takes us on a trip to see the structure of that teeny tiny atom which some brilliant scientists have already discovered, and some have even looked beyond the atoms, into the electrons, protons and neutrons.

This Story also Contains

1. NCERT Notes for Class 11 Chemistry Chapter 2
2. Properties of Cathode Rays
3. Determination of Charge by Mass Ratio for Electrons.
4. Charge on the Electron
5. Discovery of Proton Anode Rays
6. Thomson Model of The Atom
7. Rutherford Alpha Particle Scattering Experiment
8. Atomic Number
9. Mass Number
10. Isotopes and Isobars
11. Black Body Radiation
12. Planck's Quantum Theory
13. Photoelectric Effect
14. Dual Behavior of Electromagnetic Radiation
15. Line Spectrum of Hydrogen
16. Bohr's Model of An Atom
17. Quantum Mechanical Model of The Atom
18. Quantum numbers
19. NCERT Class 11 Notes Chapter-Wise
20. Subject Wise NCERT Exemplar Solutions
21. Subject Wise NCERT Solutions

This chapter explores the discovery of Dalton, Thomson, Rutherford, and Bohr and explains the concept of quantum numbers, electronic configuration, orbitals, and the dual nature of matter. To excel in board and competitive exams, students have to focus on the discovery of subatomic particles, atomic models, quantum mechanics, Line Spectra of hydrogen, etc.
To tackle the NCERT Exemplar question of the chapter Structure of Atom, our subject matter experts have meticulously structured the notes of NCERT, which perfectly aligns with the CBSE Board, NEET, JEE Main, etc. exams. The notes are written in detailed solutions with easy to follow approach. By following the solutions, students can grasp atomic models and quantum numbers, master numerical problems on energy levels, wavelength, and atomic spectra, etc.

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NCERT Notes for Class 11 Chemistry Chapter 2

Structure of Atom

Discovery of electron discharge tube experiment

In the discharge tube experiment conducted by William Crookes, the conduction of electricity through gases at low pressure is studied. The discharge tube is a cylindrical hard glass tub about 60 cm in length. Both ends of the discharge are to be sealed and fitted with the two metal electrodes.

Only at very low pressure and high voltages electric discharge through the gases is observed. The pressure of a gas can be adjusted by using evacuation. Under the supply of high voltage across the electrodes current starts flowing and the stream of particles is moving from the tuber in the negative electrode to the positive electrode. And it is called cathode rays.

Properties of Cathode Rays

• They travel in a straight line.

• Cathode rays begin from the cathode and then move towards the anode.

• They are invisible but can be made visible with the help of materials like a fluorescent or a Phosphorescent.

• Since under the supply of electric charge they are moved to the positive charge which indicates that cathode rays consist of negatively charged particles.

• The properties of cathode rays do not depend upon the material of the electrode in which it is used and the nature of gases that are present in the cathode ray tube.

Determination of Charge by Mass Ratio for Electrons.

Experimentally JJ Thomson determined the charge-to-mass ratio of the electrons. According to Thomson’s experiment, the amount of deviation for a particle from their path under the presence of an electric and a magnetic field depends upon

• Deflection tends to be higher when the magnitude of the charge is higher for the particles.

• When the mass of a particle is lighter the deflection will be greater.

• With the increase of voltage across the electrodes for with the increase of the strength of the magnetic field the deflection of electrons will also rise.

Thomson determined the value of eme=1.758820×10¹¹Ckg^-1

Where a mass of the electron and is the magnitude of charge.

Charge on the Electron

Oil drop experiment conducted by R A Milliken finds out the charge on the electron.

Charge of an electron=-1.6022×10^-19C

Mass of electronee/me=1.6022×10^-19C/1.758820×10¹¹Ckg^-1=9.1094×10^-31kg

Discovery of Proton Anode Rays

Goldstein conducted another experiment with the help of a perforated cathode ray tube. A new type of eraser passes through the perforation of the card order by reducing the pressure and it is moving just the opposite direction as that of the cathode rays. These are named canal rays or anode rays.

Properties of anode rays

• The magnitude of the positive charge of anode rays depends upon the nature of the gas that is present in the tube.

• It also depends on the gas for the value charge to mass ratio.

• The behavior of anode rays is just opposite to the cathode rays and the magnetic and electric field.

Thomson Model of The Atom

JJ Thomson proposed a structure for an atom that can be regarded as a sphere of an approximate radius carrying a positive charge due to protons and in which the negatively charged electrons are embedded into it. Thereby the atom can be visualized as a pudding for a cake positively charged with electrons in it. And the mass of atoms evenly spread over the atom. Although it was able to explain the overall neutrality of the other it cannot explain the result of the scattering experiment conducted by Rutherford.

JJ Thomson's model of atom

Rutherford Alpha Particle Scattering Experiment

By the use of a thin foil of metals like gold silver platinum with a beam of fast-moving Alpha particles Rutherford conducted some scattering experiments. And the thin gold foil has a fluorescent screen surrounding it. Whenever a particle struck the screen a flash of light was produced.

Rutherford alpha particle scattering experiment

One of the characteristics of the Rutherford Alpha scattering experiment is,

• Most of the Alpha particles do not undergo any deflection only a few of them undergo deflection through some small angles.

• Only a few deflectors back.

Based on these observations Rutherford concluded that since most of the Alpha particles passed through the foil without taking place any deflection there is much space in the atom. Since only a small question underwent deflection so the positive charger is a very small volume concentrated at the center. And the small portion of the atom is named the nucleus.

Atomic Number

The number of protons present in the nucleus is referred to as the atomic number. For example, the number of protons in a sodium atom is 11 and also the atomic number of sodium is 11. For maintaining electrical neutrality the number of electrons in an atom is equal to the number of protons.

Mass Number

The sum of the number of protons and neutrons present in a nucleus together is called the mass number.

Isotopes and Isobars

The element that is having the same atomic number but the different mass number is isotopes. ₁H¹, ₁H², ₁H³. And the element that is processing the same mass number but the different atomic number is isobars. ₆C¹⁴, ₇N¹⁴.

Development Leading to Bohr's Model of The Atom

• Radiation causes wavelike and particle-like properties which means that it is having a dual character

• The atomic spectra can be explained only by assuming the quantum state electronic energy levels in atoms.

Black Body Radiation

The ideal body that has the ability to emit and absorb all frequencies is a black body. And the corresponding radiation emitted by a black body is black body radiation.

Planck's Quantum Theory

• Max Planck put forward a theory for explaining the phenomenon of blackbody radiation and the photoelectric effect. The theory focuses on

• The radiant energy absorbed or omitted is in the form of small pockets of energy and these small pockets of energy are quantum.

• The energy of each Quantum is directly proportional to the frequency of the radiation emitted.

E= hv

Photoelectric Effect

When a beam of light passes on the surface of some metals electrons are emitted from the metal surface. This phenomenon is the photoelectric effect. And it has been observed that only photons of light of a particular frequency that is the threshold frequency can cause the photoelectric effect. The kinetic energy of the electron emitted from the surface of the metal is directly proportional to the frequency of the striking photons. And also when the intensity of the photon of the light is increasing more electrons are ejected.

Photoelectric effect

Dual Behavior of Electromagnetic Radiation

Light and other electromagnetic radiation consist of a dual nature. That is they have particle and wave-like properties. Radiations emitted with the matter show the properties of a particle lies by exhibiting propagation. Electrons also exhibit wave-particle duality.

Spectrum

When a light ray passes through a prism the wave which is having a short wavelength forms a colored band more than the one which is having a longer wavelength. These bands then spread to form a series of colored bands and are called the spectrum. The one which is deviated least is the one which is having the longest wavelength that is their red color.

Continuous spectrum

When white light is passed through a prism it will split seven different colored bands just like a rainbow and these colors are continuous and it is called the continuous spectrum.

Emission spectra

When radiation is emitted from a source and is then passed to a prism which is then received on a photographic plate it is called the emission spectra. The emission spectrum can be observed by heating a substance with a high temperature.

Line spectra

The vapors of some volatile substance when allowed to fall on the flame of a Bunsen burner and which is further analyzed with the help of spectroscopy some specific colored lines appear on the photographic plate. The colored lines differ for different substances. Sodium emits yellow light while potassium emits violet light

Absorption spectra

A ray of light, when allowed to pass through the vapors of a substance and the transmitted light, is then allowed to strike a prism dark lines appear. The dark line represents that the radiations corresponding to them are absorbed by the substance. The spectrum is called the absorption spectrum.

Line Spectrum of Hydrogen

In the discharge October under low pressure when an electric discharge is passed through the hydrogen gas light is emitted and is analyzed by spectroscopy The spectrum consisting of a large number of lines is obtained and the spectrum is called the hydrogen spectrum. The series of lines is named as Lyman series, Balmer series, Paschen series, Brackett series, and Pfund series.

Bohr's Model of An Atom

Bohr's model of the atom is based on Planck's Quantum theory. According to this model in an atom, the electron revolves around the nucleus in a definite circular path called the orbits. Each orbit has definite energy and it can be named as the energy levels. The energy orbits that are permitted for an electron are in which the angular momentum of the electron is an old multiple of h/2π. And when an electron is present in an orbit it neither losses nor absorbs energy it will remain in a constant form. But when energy is supplied to an electron it will jump into a higher energy state that is the excited state.

Achievements of Bohr's theory

• It has been explained the stability of an atom.

• It also helps for calculating the energy of an electron in a hydrogen atom and also for one-electron species.

Quantum Mechanical Model of The Atom

• Theoretical science that deals with the study of the motion of microscopic objects that have both particle-like and wave-like properties.

• The energy of an electron in an atom is in a quantized state.

• Due to the wavelike properties of electrons, it will exist in the quantized electronic energy level.

• The exact position and exact velocity of an electron in an atom cannot be found out simultaneously.

• For an atomic orbital, it has a wave function

• The square of the orbital wave function 2 is the probability of finding an electron.

Quantum numbers

As the atomic energy levels or the orbits are quantized that can be expressed in the terms of quantum number. Quantum numbers are;

• Principal
• Azimuthal
• Magnetic
• Spin

Principal quantum number(n)

It represents the principal shell of an atom. It can have integral values except zero like 1,2,3,.... Also denoted as K,L,M,.....etc.

Maximum number of electrons in a principal shell can be 2n2 where n is principal quantum number.

This quantum number gives information about :

• Distance of electron from nucleus i.e., size of electron cloud.
• Energy of electron in any shell

${\mathrm{E}}_{\mathrm{n}}=-\frac{1312\times {\mathrm{Z}}^2}{{\mathrm{n}}^2}\mathrm{kJ}/\mathrm{mol}$

Where Z is atomic number and n is principal quantum number.

Azimuthal quantum number(l):

Azimuthal quantum number represents the subshell or subenergy shell in an atom.

l has values from 0 to (n-1).

For eg: for n=2 ; l= 0, 1

Subshell notation for 0, 1 is s and p.

No. of electrons [2(2l+1)]: for s subshell = 2; for p subshell = 6.

Magnetic quantum number(m):

It represents the number of orbitals present in a subshell.

m has values ranging from -l to +l including zero.

For eg: for ‘s’ subshell :

1. The value of l is 0

2. m has value=0

For ‘p’ subshell :

1. The value of l is 1

2. m has value= -1, 0, +1

Spin quantum number(s):

Electrons in an orbital can spin either clockwise or anticlockwise.

Thus, an electron can have only two possible values of this quantum number, either $+\frac{1}{2}$ or $-\frac{1}{2}$ respectively.

NCERT Class 11 Notes Chapter-Wise

Subject Wise NCERT Exemplar Solutions

Subject Wise NCERT Solutions