CBSE Class 9 Chemistry Structure Of An Atom Notes Set A

Download CBSE Class 9 Chemistry Structure Of An Atom Notes Set A in PDF format. All Revision notes for Class 9 Chemistry have been designed as per the latest syllabus and updated chapters given in your textbook for Chemistry in Standard 9. Our teachers have designed these concept notes for the benefit of Grade 9 students. You should use these chapter wise notes for revision on daily basis. These study notes can also be used for learning each chapter and its important and difficult topics or revision just before your exams to help you get better scores in upcoming examinations, You can also use Printable notes for Class 9 Chemistry for faster revision of difficult topics and get higher rank. After reading these notes also refer to MCQ questions for Class 9 Chemistry given our website

CBSE Class 9 Chemistry-Structure of an Atom (2). Learning the important concepts is very important for every student to get better marks in examinations. The concepts should be clear which will help in faster learning. The attached concepts made as per NCERT and CBSE pattern will help the student to understand the chapter and score better marks in the examinations. 



1. Energy of electron in H-atom, En = -2.18 x 10 -18 / n2 Jatom-1

2. Energy of electron in H-like species atom, En = -2.18 x 10 -18 z2 / n2 Jatom-1

3. Angular momentum of electron, mvr = nh/2π

4. de- Broglies wavelength, λ = h / mv or λ = h / p

5. Plank’s equation E = hϑ

6. Einstein equation , E = mc2

7. Wave number( )=1/ λ = RH[1/n12 – 1/n22]

8. Isotopes: These are the atoms of the same element having the same atomicnumber but different mass number.e g 1H1,1H2,1H3

9. Isobars: Isobars are the atoms of different elements having the same massnumber but different atomic number.

e g 18Ar40 ,20Ca40 

10. Isoelectronic species: These are those species which have the same numberof electrons.

11. Electromagnetic spectrum:-It is a continuous spectrum. It consists of a range of electromagnetic radiations arranged in the order of increasing wavelengths or decreasing frequencies.

12. Spectroscopy:-The study of emission or absorption spectra is referred as spectroscopy.

13. Emission spectrum: The spectrum of radiation emitted by a substance that has absorbed energy is called an emission spectrum.

14. Absorption spectrum : The spectrum obtained when radiation is passed through a sample of material. The sample absorbs radiation of certain wavelengths. The wavelengths which are absorbed are missing and come as dark lines.

15. Bohr’s model for hydrogen atom:

a. An electron in the hydrogen atom can move around the nucleus in a circular path of fixed radius and energy. These paths are called orbits orenergy levels. These orbits are arranged concentrically around the nucleus.

b. As long as an electron remains in a particular orbit, it does not lose or gain energy and its energy remains constant.

c. When transition occurs between two stationary states that differ in energy, the frequency of the radiation absorbed or emitted can be calculated

d. An electron can move only in those orbits for which its angular momentum is an integral multiple of h/2π The radius of the nth orbit is given by rn =52.9 pm x n2 /Z energy of electron in nth orbit is ;

CBSE Class 9 Chemistry-Structure of an Atom (2)_1

16. Black body: An ideal body, which emits and absorbs all frequencies, is called a black body. The radiation emitted by such a body is called black body radiation.

CBSE Class 9 Chemistry-Structure of an Atom (2)_2

17. Photoelectric effect: The phenomenon of ejection of electrons from the surface of metal when light of suitable frequency strikes it is called photoelectric effect. The ejected electrons are called photoelectrons.

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18. Threshold frequency (vo): For each metal there is a characteristic minimum frequency below which photoelectric effect is not observed. This is called threshold frequency.

19. Heisenberg uncertainty principle: It is impossible to determine simultaneously the exact position and exact momentum of an electron. Mathematically, ∆x ∆p ≥ h/ 4 π\

20 .Quantum number’s: These are the numbers obtained by solution of Schrodinger equation used to define the energy of electron its position, orientation and spinning.

21. There are four quantum numbers namely principal, azimuthal, magnetic and spin quantum no.

22. Bohr burry’s rule:- The lower the value of (l) for an orbital, the lower is its energy. If two orbitals have the same value of (l), the orbital with lower value of n will have the lower energy.

23. Degenerate orbitals- The orbitals having the same energy are called degenerate orbitals.

24. Aufbau principle: electron filled in various subshell in increasing order of their energies.

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25. Pauli’s exclusion principle: No two electrons in an atom can have same set of four quantum numbers.

26. Hunds rule of maximum multiplicity: Paring of electrons in the orbitals belonging to the same sub-shell does not take place until each orbital belonging to that sub-shell has got one electron each.

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Stability of completely filled and half filled subshells:

a. Symmetrical distribution. 

b. Exchange energy

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