# CBSE Class 11 - Chemistry - Chapter - Structure Of Atom - Important Points To Remember

Structure of Atom

1. Discovery of Electron — Discharge Tube Experiment:

In 1879, William Crooks studied the conduction of electricity through gases at low pressure. He performed the experiment in a discharge tube which is a cylindrical hard glass tube about 60 cm. in length.

Mass of electron = 9.109534 x 10⁻³¹ kg

Charge on a electron = (-) 1.602 x 10⁻¹⁹ C

2. Properties of Cathode Rays

(i) Cathode rays travel in straight line.

(ii) Cathode rays start from cathode and move towards the anode.

3. Properties of Anode Rays

(i) The value of positive charge (e) on the particles constituting anode rays depends upon the nature of the gas in the discharge tube.

(ii) The charge to mass ratio of the particles is found to depend on the gas from which these originate.

4. Proton: The smallest and lightest positive ion was obtained from hydrogen and was called proton.

Mass of proton = 1.676 x 10⁻²⁷ kg

Charge on a proton = (+) 1.602 x 10⁻¹⁹ C

5. Neutron: It is a neutral particle. It was discovered by Chadwick (1932).

Mass of neutron = 1.675 x 10⁻²⁷ kg

6. Rutherford’s Nuclear Model of an Atom

(i) The positive charge and most of the mass of the atom was densely concentrated in an extremely small region. This very small portion of the atom was called nucleus by Rutherford.

(ii) The nucleus is surrounded by electrons that move around the nucleus with a very high speed in circular paths called orbits.

(iii)  Electrons and nucleus are held together by electrostatic forces of attraction.

7. Bohr's Model

(i) In an atoms, the electrons revolve around the nucleus in a fixed orbit of fixed radii having fixed velocity.

(ii) Each orbit is associated with  definite energy and therefore these are known as energy shells

K, L, M, N, O

1   2   3    4   5

(iii) Angular momentum of electrons in an atom is quantised.

8. Atomic Number (z): The number of protons present in the nucleus is equal to the atomic number (z). For example, the number of protons in the hydrogen nucleus is 1, in sodium atom it is 11, therefore, their atomic numbers are 1 and 11.

9. Mass Number: Number of protons and neutrons present in the nucleus are collectively known as nucleons.

The total number of nucleons is termed as mass number (A) of the atom. :

Mass Number (A) = Number of protons (p) + Number of neutrons (n).

= Atomic Number (z) + Number of neutrons (n).

10. Isotopes: Atoms with identical atomic number but different atomic mass number are known as Isotopes.

11. Isobars: Isobars are the atoms with same mass number but different atomic number.

For example, ¹⁴₆C and ¹⁴₇N are isobars.

Another example is ⁴⁰₁₈Ar, ⁴⁰₁₉K and ⁴⁰₂₀C are typical isobars.

Each of these have same mass number but different atomic number.

12. Electromagnetic radiations: Energy emitted from any source (in forms of waves) in which electric and magnetic fields oscillated perpendicular to each other and travelling with a velocity of light is known as EM radiation.

13. Wave Length: It is defined as the distance between any two consecutive crests or troughs. It is represented by λ and its S.I. unit is metre.

1A°=10⁻¹⁰m.

A° = Angstrom

14. Frequency: Frequency of a wave is defined as the number of waves passing through a point in one second. It is represented by ν (nu) and is expressed in Hertz (Hz).

1 Hz = 1 cycle/sec.

15. Velocity: Velocity of a wave is defined as the linear distance travelled by the wave in one second. It is represented by c and is expressed in cm/sec or m/sec.

c = ν λ

where c is the velocty of light.

16. Amplitude: Amplitude of a wave is the height of the crest or the depth of the trough. It is represented by ‘A’ and is expressed in the units of length.

17. Wave Number: It is defined as the number of waves present in 1 cm length. Evidently it will be equal to the reciprocal of the wave length. It is represented by    (read as nu bar)

v   = 1/λ

18. Electromagnetic Spectrum: When electromagnetic radiations are arranged in order of their increassing wave lengths or decreasing frequencies, the complete spectrum obtained is called electromagnetic spectrum.

19. Energywise order for EM radiation:

Cosmic rays < γ rays < X rays < UV < VIBGYOR < IR < Microwaves < Radiowaves

20. Photon: A packet or particle of light energy is knows as Photon.

21. Planck’s quantum theory: The energy emitted or absorbed by a source is discontinuous in form of small packet of energy, called quantum.

Quantum of light is called photon.

E ∝ v

E = hv (h = Planck's constant)

E = nhv (h = 6.626×10⁻³⁴ J sec)

If ‘n’ photons are emitted,  E = nhυ

22. Photoelectric Effect: Hertz, in 1887 discovered that when a beam of light of certain frequency strikes the surface of some metals, electrons are emitted or ejected from the metal surface. The phenomenon is called photoelectric effect.

hυ – hυ₀ = (1/2)mv²

hυ ⇒ Energy of incident light on metal surface.

hυ₀ ⇒ Work function of metal.

(1/2)mv² = Kinetic energy by which e– is emitted from metal surface

23. Quantum Numbers: Atomic orbitals can be specified by giving their corresponding energies and angular momentums which are quantized (i.e., they have specific values). The quantized values can be expressed in terms of quantum number. These are used to get complete information about electron i.e, its location, energy, spin etc.

24. Magnetic Orbital Quantum Number (m or m₁): The magnetic orbital quantum number determines the number of preferred orientations of the electrons present in a subshell. Since each orientation corresponds to an orbital, therefore the magnetic orbital quantum number determines the number of orbitals present in any subshell.

25. Spin Quantum Number (S or m₈): This quantum number helps to explain the magnetic properties of the substances. A spinning electron behaves like a micromagnet with a definite magnetic moment. If an orbital contains two electrons, the two magnetic moments oppose and cancel each other.

26. de Broglie equation: All material particles in motion also exhibit wave like properties.

λ = h / (mv) = h / p

For microscopic particles mass is very less therefore Wavelength of wave associated with it can be detected.

27. Heisenberg's Uncertainty Principle: It is impossible to determine the exact position and velocity of a moving subatomic particle simultaneously with accuracy.

28. Aufbau principle

(a) Electrons are filled in increasing order of energy of sub-shell.

(b) As ‘n + l’value increases energy of e– increases in that sub-shell.

(c) For two sub-shells with same ‘n + l’ value, as ‘n’ value increases energy of e– increases.

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

30. Hund’s rule of maximum multiplicity: The pairing of e– in degenerate orbitals (different orbitals with same energy) will get paired only once they have been singly occupied with same spin.