CBSE Class 11 Chemistry Notes: Structure of Atom (Part  II)
This article provides you the revision notes on Class 11 Chemistry: Chapter Structure of Atom. Some of the important topics covered in this part are: Bohr’s model for hydrogen atom, Dual behavior of matter, Heisenberg’s Uncertainty Principle, Quantum Mechanical Model of The Atom, electronic configuration of an atom.
This article provides you the revision notes on Class 11 Chemistry: Chapter Structure of Atom, to give you a quick glance of the chapter. This article is a continuation of the revision notes on Class 11 Chemistry, Chapter Some Basic Concepts of Chemistry, PartI. In PartI you got to learn about atom and its subatomic particles, Rutherford’s nuclear model of atom, Thomson’s model of atom, electromagnetic radiations and electromagnetic spectrum.
CBSE Class 11 Chemistry Notes: Structure of Atom (Part  I)
Topics covered in this part of chapter notes for ‘Structure of Atom’, are:
 Bohr’s model for hydrogen atom
 Dual behavior of matter
 Heisenberg’s Uncertainty Principle:
 Quantum Mechanical Model of The Atom
 Probability density:
 Nodes
 Aufbau Principle:
 Pauli Exclusion Principle
 Hund’s rule of maximum multiplicity
 Stability of completely filled and half filled subshells
These quick notes are prepared strictly according to the latest CBSE syllabus for Class 11.
Key notes of the chapter are:
Bohr’s model for hydrogen atom (By Neil Bohar in 1913):
Postulates for Bohr’s model are:
1. Electron in hydrogen atom moves around the nucleus in circular path of fixed radius and energy. These paths are called orbits or energy levels.
2. As long as an electron remains in a particular orbit, it does not lose or gain energy and its energy remains constant.
3. However, when electron will move from a lower stationary state to a higher stationary state a certain amount of energy is absorbed by the electron or some energy is emitted when electron moves from higher stationary state to lower stationary state
4. Frequency of radiations emitted or absorbed when transition of an electron occurs, is given by
Where, E_{1} & E_{2} represent the lower & higher energy states respectively.
5. An electron can move only in those orbits for which its angular momentum is an integral multiple of h/2π, i.e.,
[Where n =1,2,3.....]
Some more assumptions of Bohar’s model are:
1. The radii of the stationary states are expressed as:
2. Energy of an electron in nth orbit is given as:
Limitations of Bohr’s model of atom:
1. It failed to account for the finer details of the hydrogen spectrum.
2. It was unable to explain spectrum of atoms containing more than one electron.
3. It failed to explain splitting of the spectral lines in presence of electric (Stark effect) or magnetic field ( Zeeman effect).
4. It failed to explain formation of molecules from atoms by chemical bonding.
Dual behavior of matter:
de Broglie proposed that matter exhibits dual behavior, i.e., matter shows both particle as well as wave nature and gave the following relation between wavelength (λ) and momentum (p) of a material particle.
[Where, m is the mass of the particle, v its velocity and p its momentum]
The above equation is named as de Broglie’s relation.
Heisenberg’s Uncertainty Principle:
It states that it is impossible to determine simultaneously, the exact position and exact momentum (or velocity) of an electron. The product of their uncertainties is always equal to or greater than h/4π. I.e.,
Where, Δx = uncertainty in position
Δp = uncertainty in momentum
Significance of Uncertainty Principle
 Heisenberg’s uncertainty principle rules out the existence of definite paths or trajectories of electrons and other similar particles
 The effect of Heisenberg Uncertainty Principle is significant only for motion of microscopic objects and is negligible for that of macroscopic objects.
Reasons for the Failure of the Bohr Model:
1. It ignores the dual behavior of matter.
2. It contradicts Heisenberg’s uncertainty principle.
Quantum Mechanical Model of Atom
 Branches of science which explain duel behavior of Metter is called quantum mechanics .
 It is based on a fundamental equation which is called Schrodinger wave equation.
Try the following questions to check your preparedness for the above stated topics:
1. Why are Bohar's orbits called stationary states?
2. Show that the ciccumference of Bohar orbit for the hydrogen atom is an integral multiple of the Broglie wavelength associated with the electron revolving around the nucleus.
3. Why is the uncertainty principle not applicable to macroscopic particles?
4. Calculate the uncertainty in the position of a particle when the uncertainty in the momentum is (a) 1 × 10^{2} (b) zero.
5. What is the significance of the statement "Product of uncertainity in position and momentum is always constant if measured simultaneously"?
6. Why is the uncertainty principle not applicable to macroscopic particles?
CBSE Class 11 Chemistry Notes: Some Basic Concepts of Chemistry (Part  I)
CBSE Class 11 Chemistry Notes: Some Basic Concepts of Chemistry (Part  II)
Hydrogen Atom and the Schrödinger Equation:
These quantized energy states and corresponding wave functions which are characterized by a set of three quantum numbers (principal quantum number n, azimuthal quantum number l and magnetic quantum number ml ) arise as a natural consequence in the solution of the Schrödinger equation.
Probability density:
 ψ gives us the amplitude of wave.
 The wave function is a mathematical function whose value depends upon the coordinates of the electron in the atom and does not carry any physical meaning.
 Ψ^{2 }gives us the region in which the probability of finding an electron is maximum. It is called probability density.
Orbital:
The region of space around the nucleus where the probability of finding an electron is maximum is called an orbital.
Quantum numbers:
There are a set of four quantum numbers which specify the energy, size, shape and orientation of an orbital.
(i) Principal quantum number (n):
 It identifies shell, determines sizes and energy of orbitals.
n 
1 
2 
3 
4 
Shell 
K 
L 
M 
N 
Total number of orbitals in a shell = n^{2} 
1 
4 
9 
16 
Maximum number of electrons in a shell = 2n^{2} 
2 
8 
18 
32 
(ii) Azimuthal quantum number (l):
 It is also known as orbital angular momentum or subsidiary quantum number.
 It identifies subshell, determines the shape of orbitals, energy of orbitals and orbital angular momentum of an electron, which is given as,
 The number of orbitals in a subshell = 2l + 1.
 For a given value of n, l can have n values ranging from 0 to n – 1, that is, for a given value of n, the possible value of l are : l = 0, 1, 2, .......... (n–1)
n 
l 
Subshell Notation 
n 
l 
Subshell Notation 
1 
0 
1s 
4 
0 
4s 
2 
0 
2s 
4 
1 
4p 
2 
1 
2p 
4 
2 
4d 
3 
0 
3s 
4 
3 
4f 
3 
1 
3p 



3 
2 
3d 



(iii) Magnetic quantum number (m_{l}):
 It is also known as magnetic orbital quantum number.
 Itgives information about the spatial orientation of the orbital.
 Number of orbitals in ecah subshell = 2l+1. i.e., for each value of l, ml = – l, – (l –1), – (l–2)... 0,1...(l – 2), (l–1), l
(iv) Electron spin quantum number (m_{s}):
 It refers to orientation of the spin of the electron.
 It can have two values +1/2 and −1/2. +1/2 identifies the clockwisespin and −1/2 identifies the anti clockwise spin.
Nodal surfaces or nodes:
The region where the probability density function reduces to zero is called nodal surfaces or simply nodes.
Radial nodes: Radial nodes occur when the probability density of wave function for the electron is zero on a spherical surface of a particular radius. Numberof radial nodes = n – l – 1
Angular nodes: Angular nodes occur when the probability density wavefunction for the electron is zero along the directions specified by a particular angle. Number of angular nodes = l
Total number of nodes = n – 1
Degenerate orbitals:
Orbitals having the same energy are called degenerate orbitals. Shape of p and dorbitals
Shielding effect or screening effect:
Due to the presence of electrons in the inner shells, the electron in the outer shell will not experience the full positive charge on the nucleus. So, due to the screening effect, the net positive charge experienced by the electron from the nucleus is lowered and is known as effective nuclear charge. Effective nuclear charge experienced by the orbital decreases with increase of azimuthal quantum number (l).
CBSE Class 11 Chemistry Syllabus 2017 – 2018
Aufbau Principle:
 According to this principle in the ground state of the atoms the orbital’s are filled in order of their increasing energies means electrons enter higher energy orbital’s so order in which orbital’s are filled is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p.
Pauli Exclusion Principle:
 Two electrons in an atoms can’t have same set of four quantum numbers.
 Only two electrons may exist in same orbital and these electrons must have opposite spin.
Hund’s rule of maximum multiplicity:
 Pairing of electrons in the orbitals belonging to the same subshell (p, d or f) does not take place until each orbital belonging to that subshell has got one electron each i.e., it is singly occupied.
Electronic configuration of atoms:
 It refers to the arrangement of electrons in different orbitals of an atom.
 The electronic configuration of different atoms can be represented in two ways.
(i) s^{a}p^{b}d^{c}...... notation.
(ii) Orbital diagram in which each orbital of the subshell is represented by a box and the electron is represented by an arrow (↑)a positive spin and an arrow (↓) a negative spin.
Stability of completely filled and half filled subshells:
 Symmetrical distribution of electrons: the completely filled or half filled subshells have symmetrical distribution of electrons in them and are more stable.
 Exchange energy: The two or more electrons with the same spin present in the degenerate orbitals of a subshell can exchange their position and the energy released due to this exchange is called exchange energy. The number of exchanges is maximum when the subshell is either half filled or completely filled. As a result the exchange energy is maximum and so is the stability.
Try the following questions to check your preparedness for the above mentioned topics:
1. What physical menaing is attributed to the square of the absolute value of wave function Ψ^{2} ?
2. Write electronic configuration of Cr^{3+} ion.
3. State Pauli's exclusion principle.
4. How many nodes are there in 3s orbital?
5. What is teh value of orbital angular momentum of 6s orbital?
6. What is meant by degenerate orbitals? Illustrate with the help of an example.
CBSE Class 11 Physics Syllabus 2017  2018
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