The NCERT Class 12 Physics chapter 12 notes covers a brief outline of the NCERT chapter Atoms. The main topics covered in Atoms Class 12 notes are DALTON’S ATOMIC THEORY, Thomson atomic model, Rutherford Nuclear Model of Atom, Atomic Spectra, Spectral Series, Types of Spectral Series, Bohr Model of Hydrogen Atom, Energy of Orbits, Drawbacks of Bohr Model and De-Broglie Hypothesis.

The basic equation can be learned in Class 12 Physics chapter 12 notes but the important derivations are not included in the CBSE Class 12 Physics chapter 12 notes.

Also, students can refer,

• NCERT Notes Class 12 Physics
• NCERT Solutions for Class 12 Physics Chapter 12 Atoms
• NCERT Exemplar Class 12 Physics Chapter 12 Solutions Atoms

NCERT Class 12 Physics Chapter 12 Notes

Atoms are the fundamental building blocks of matter with a positively charged nucleus at the center. It is surrounded by negatively charged electrons. Every object whether inorganic, organic, or even synthetic is made up of atoms.

There were many atomic models given initially.

DALTON’S ATOMIC THEORY:

• According to Dalton’s Atomic Theory:-

• Atoms are the tiniest constituents of matter and can’t be divided (indivisible).

• Atoms of different elements differ in mass and size.

• Atoms are reoriented in an exceedingly chemical action (not generated or destructed).

Merits:

• It follows the laws of:

a) conservation of mass,

b) of fixed composition

c) of multiple proportions

Demerits:

• Electrostatics experiments were not demonstrated by it (dry paper bits sticking to comb) that showed that charge exists.

THOMSON MODEL OF ATOM:

According to Thomson:-

• Atom is like a sphere where a positive charge is uniformly distributed.

• Electrons are scattered inside the atom in such a way that the most stable electrostatic arrangement is achieved. It means that the minimum possible energy of the system should be achieved.

• It is also known be the watermelon model or plum pudding model or raisin bread model.

• It illustrates that the net charge on an atom is neutral (equal positive and negative charges).

• It was not consistent with the experiments conducted later and the discovery of neutrons and protons.

RUTHERFORD’S NUCLEAR MODEL OF ATOM:

• Two scientists Geiger and Marsden carried out a few experiments on the advice of Rutherford.

• On a very thin gold foil, a highly energetic ray of α-particles was passed that are positively charged with energy of 5.5MeV.

• It scattered α-particles when it was stroked on a zinc sulphide screen (surrounding the thin gold foil) and produced light flashes (scintillations) which were observed with the help of a microscope.

• The distribution of scattered α-particles was analyzed and plotted as a graph as a function of scattering angle (θ).

• As the scattering angle (θ) got higher, the count of scattered α-particles was observed to be lower, and vice versa.

• Almost all of the α-particles passed through the gold foil without being deflected, and an infinitesimally small number of α particles get deflected.

• From this, it was concluded that almost all the space in an atom is empty (9999999999996%). The charge is confined to a tiny region as very less variety of the charged α -particles were repelled by foil proving charge is confined to a little region. This region was referred to as the nucleus.

• Atoms have a structure almost like our scheme wherever the nucleus (like the sun) is at the centre and every one of the electrons (like planets) are revolving around it in a very circular path with massive speeds.

• Electrons and protons are certain along by electricity forces of attraction. The atom is electrically neutral.

• Rutherford was the first to discover that an atom has a nucleus. This model was called Rutherford’s nuclear model of atoms.

Electron Orbit:

The electrons orbit around the nucleus and are held together to the nucleus by the electrostatic force of attraction. The centripetal force (Fc) is provided by the electrostatic force (Fe) to keep the electrons in the orbit

Here r=radius of orbit,

v = velocity of orbiting electron,

e = charge of an electron,

m = mass of an electron,

Z = atomic mass of the atom,

ε₀= permittivity of free space

On solving, we get:

• Kinetic Energy(K):

putting the value of mv², we get:

• Potential Energy(U):

using the electrostatic potential between 2 charged bodies, we get:

Here negative signs show that there is a force of attraction and energy has to be given to the system to overcome this force of attraction.

• Total Energy(T):

Some important relations to note:

Kinetic energy (K) = -(1/2)Potential energy(U)

Kinetic energy (K) = -Total energy(T)

Potential energy(U) = 2×Total energy(T)

Drawbacks of Rutherford’s Model:

• Accelerated charged particle produces electromagnetic waves (Maxwell Theory), so the orbital radius of electrons should be decreasing. Also, the electron should fall into the nucleus. But as the atoms are stable in nature. This stability of atoms was not clarified by Rutherford’s model.

• The model does not give information about the electronic structure of atoms, i.e. electrons orientation, the motion of orbit and the relative energy of the electron in different orbits.

• The dual character of electromagnetic radiation is also not elaborated by the model.

ATOMIC SPECTRA:

• When an electron jumps amongst different energy levels in an atom, energy is emitted or absorbed within the style of radiation. These radiations square measure to blame for spectral lines of frequencies (or wavelength) related to the associated atom and square measure referred to as atomic spectra.

• Spectral lines square measure the brilliant associated dark line series that represent the spectrum and square measure related to an atom.

• An atom includes separate spectra wherever there exists a hard and fast specific line of energy transition of lepton with separate energy gaps. It's additionally referred to as quantity spectra.

• Another variety of spectra is continuous spectra wherever there are no specific lines of energy transition of electrons. It's the reverse of separate spectra.

The 3 types of atomic spectra are:

a) emission spectra

b) absorption spectra

c) continuous spectra

Emission spectra:

• In this case, the radiation spectrum is produced due to the absorption of energy by matter.

• When the associated electron of an atom, molecules or ions get to the next energy level than their ground or stable state because of radiation absorption, they're aforementioned to be excited.

• The emission spectrum is created once energy is provided to a sample through heating or irradiation. The wavelength or frequency of radiation emitted by the sample is determined as a performance of energy.

Absorption spectra:

• It is the alternative to emission spectra.

• Continuous radiation (energy) is directed through a sample that solely absorbs bound radiation of a selected wavelength and also the remaining spectrum is recorded. Absorbed wavelengths correspond to the dark areas of the spectrum.

• Whatever is absent (shown by dark lines) within the spectrum of the associated atom is truly a gift (shown by bright lines) within the spectrum of that atom.

Continuous spectra:

• It is fashioned once a ray of white lightweight has a prism or water droplets inflicting a continual spectrum of visible radiation of various wavelengths.

• There aren't any distinct lines or separation between any two adjacent wavelengths.

• Speed of sunshine changes with reference to the medium through which it passes. Because the medium changes, lightweight with the longest wavelength i.e. red deviates the smallest amount and on the contrary, the sunshine with the shortest wavelength i.e. violet deviates the foremost. red deviates the least and on the contrary, the light with the shortest wavelength i.e. violet deviates the most.

SPECTRAL SERIES:

• When an electric arc has atomic number 1 gas, atomic number 1 molecules would dissociate giving rise to excited (highly energetic) atomic number 1 atoms emitting radiation of that frequency, whereas returning to its state.

• Hydrogen spectra contain five series of spectrum named after their discoverer (Lyman, Balmer, Paschen, Bracket and Pfund series) .

TYPES OF SPECTRAL SERIES:

Balmer Series:

• Balmer was the first scientist to discover a spectral series of hydrogen atoms.

• It contains a visible radiation spectrum.

• It was experimentally found that these spectral lines could be expressed mathematically in the form of wavelength.

Where

R = Rydberg constant = 109677cm^-1,

n=3, 4, 5…… (higher discrete energy state from which electron jumps to 2^nd energy state thus emitting radiation)

λ = wavelength of radiation in (cm)

• For maximum wavelength(λ_max) in the Balmer series, n=3 (has to be minimum):

• For minimum wavelength (λ_min) in the Balmer series

Lyman Series:

• These are the spectral series when radiation emitted is a result of jumping electrons from higher energy states to ground states.

Mathematically it is expressed as

Where n=2, 3, 4.....

• For maximum wavelength in the Lyman series, n=2 (has to be minimum):

• For minimum wavelength in the Lyman series

Similarly, other series can also be expressed as:

Paschen Series:

Mathematically it is expressed as

where n=4, 5, 6....

Bracket Series:

Mathematically it is expressed as

where n=5, 6, 7.....

Pfund Series:

Mathematically it is expressed as

where n=6, 7, 8....

• Atomic spectra has a wide scope in the study of electronic structures of various atoms, molecules and ions.

• Elements have their different spectral series. Spectral series is used in the identification of unknown atoms.

• Many elements were discovered with the help of spectroscopic methods, such as Rubidium (Ru), Caesium (Cs), Helium (He), Gallium (Ga), Thallium (Tl), Scandium (Sc).

BOHR’S MODEL OF HYDROGEN ATOM:

• Drawbacks of Rutherford’s atomic model lead to the Bohr’s Model where he came up with 3 postulates:

• First Postulate: Atoms have specific stable energy states called stationary states. Here the electrons could orbit around the nucleus without emitting any radiation.

• Second Postulate: The electrons only move in the orbits called stable orbits. Here the angular momentum (L) of electrons is equal to the integral multiples of h/2π. This leads to the quantization of moving electron.

Where,

h= Planck’s constant

L_N= angular momentum of the electron in orbit

v_N= velocity of the electron in n^th orbit

R_N= radius of n^th orbit

n= permitted orbits on which electrons revolve

• Third Postulate: When electrons jump from a higher (initial) energy state to a lower (final) energy state, emit a photon of energy which is equal to the energy difference between the 2 energy states. Its frequency is given by:

Bohr’s Radius: The radius on which electron moves around the nucleus in the orbit is described by Bohr’s model. It is known as Bohr’s radius.

Using the second postulate and Rutherford’s model

Using the value of from both the equations, we get

On solving (and putting Z=21 for hydrogen atom) we get:

For the radius of the innermost orbit, putting n=1

The velocity of Electron in the Orbit:

Energy of Orbits:

The orbital energy possessed by orbiting electrons in the discrete energy levels in Bohr’s model is known as the energy of orbits.

We also know from Rutherford’s Model that total energy (T) is given by

By putting the value of Bohr’s radius, we get

By putting the values of electron mass (m), charge (e), the permittivity of free space (ε₀), Planck’s constant (h), we get,

DRAWBACKS OF BOHR’S MODEL:

• It was basically for hydrogen atoms.

• It was unable to elaborate spectra of multi-electron atoms.

• The wave nature of electrons wasn't even by the model.

• It was unable to illustrate the molecules making process of chemical reactions.

• It also violate Heisenberg’s Principle which says that it was impossible to evaluate the precise position and momentum of electron simultaneously. Only their probability could be calculated.

DE-BROGLIE’S HYPOTHESIS:

• De Broglie’s Hypothesis showed the wave-particle nature of matter.

• It explained that like photons, electrons should even have mass or momentum and wavelength.

• It is valid just for the subatomic (microscopic) particles like electrons, protons etc. Here mass is incredibly little, so wavelengths are massive enough to be discernible through an experiment.

• It is not valid for the macroscopic particles since their mass is very large resulting in the wavelength to be too small to be experimentally observable.

Significance of NCERT Class 12 Physics Chapter 12 Notes

The chapter, Atoms Class 12 notes will be very helpful in order to revise the chapter and to get an idea about the main topics covered in it. Also, this NCERT Class 12 Physics chapter 12 notes are very useful in covering the main topics of the Class 12 CBSE Physics Syllabus and also for competitive exams like VITEEE, BITSAT, JEE Main, NEET, etc. Class 12 Physics chapter 12 notes pdf download can be referred to prepare both in online and offline mode.

NCERT Class 12 Notes Chapterwise

Subject Wise NCERT Exemplar Solutions

• NCERT Exemplar Class 12 Solutions
• NCERT Exemplar Class 12 Maths
• NCERT Exemplar Class 12 Physics
• NCERT Exemplar Class 12 Chemistry
• NCERT Exemplar Class 12 Biology

Subject Wise NCERT Solutions

• NCERT solutions for class 12 mathematics
• NCERT solutions for class 12 chemistry
• NCERT solutions for class 12 physics

• NCERT solutions for class 12 biology

NCERT Books and Syllabus