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:
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
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
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:
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):
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):
n 
l 
Subshell Notation 
n 
l 
Subshell Notation 
1 
1s 
4 
4s 

2 
2s 
4 
1 
4p 

2 
1 
2p 
4 
2 
4d 
3 
3s 
4 
3 
4f 

3 
1 
3p 



3 
2 
3d 



(iii) Magnetic quantum number (m_{l}):
(iv) Electron spin quantum number (m_{s}):
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:
Pauli Exclusion Principle:
Hund’s rule of maximum multiplicity:
Electronic configuration of atoms:
(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:
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.
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