1) One of the fundamental particles in a substance is atom.
2) Everything around us is composed of atoms.
3) The sub atomic particles present in atom are Electrons, Protons and Neutrons.
4) The first atomic model was proposed by J.J. Thomson.
5) There are seven colours namely Violet, Indigo, Blue, Green, Yellow, Orange and Red (VIBGYOR) in a rainbow.
6) A vibrating electric charge creates a change in the electric field. The changing electric field creates a changing magnetic field. This process continues, with both the created fields being perpendicular to each other and at right angles to the direction of propagation of the wave is known as electro magnetic wave.
7) Visible light is an electro magnetic wave and the speed of light (C) is 3 × 10⁸ m/sec.
8) The distance from one wave peak to the next is called wavelength (λ) of the wave.
9) The number of wave peaks that pass by a given point per unit time is called frequency (υ) of a wave.
10) The relation between these quantities is given by c = υλ.
11) As the frequency increases, the wavelength becomes smaller.
12) The entire range of electro magnetic wave frequencies is known as the electromagnetic spectrum.
13) The familiar example of the visible spectrum in nature is the formation of a rainbow.
14) These colours, that the human eye is sensitive to, are called visible light.
15) The range of wave lengths covering red colour to violet colour is called the visible spectrum.
16) The electromagnetic spectrum consists of a continuous range of wavelengths of gamma rays at the shorter wavelength to radio waves at the longer wavelength. But our eyes are sensitive only to visible light.
17) Max plank proposed that, substances absorb or emit energy in the form of small energy packets are called quanta.
18) The energy of quantum is,
        
      where E = Energy of quantum
       h = Planck's constant = 6.626 × 10⁻³⁴ J.S.
       υ = Frequency of radiation.

19) Max plank, broke with the 'continuous energy' tradition of electro magnetic energy by assuming that the energy is always emitted in multiples of hυ.
e.g.: 1 hυ, 2 hυ, 3 hυ ...... n hυ.
20) Sodium vapours produce yellow light in street lamps.
21) Cupric chloride produces a green colour flame while Strontium chloride produces a crimson red flame.
22) Each element emits its own characteristic colour. These colours correspond to certain discrete wave lengths of light and are called line spectra.
23) The lines in atomic spectra can be used to identify unknown atoms.
24) Bohr's atomic model
(i) Neils bohr proposed that electrons in an atom occupy stationary orbits of fixed energy at different distances from the nucleus.
(ii) When an electron jumps from a lower energy state to higher energy states it absorbs energy or emits energy when such a jump occurs from a higher energy state to a lower energy state.
(iii) The energies of an electron in an atom can have only certain values E₁, E₂, E₃ ............., that is the energy is quantized.

(iv) The states corresponding to these energies are called stationary states and the possible values of the energy are called energy levels.
(v) Bohr's model explains all the line spectra observed in the case of hydrogen atom.
(vi) Bohr's model failed to account for splitting of line spectra.
25) Bohr - Sommerfeld model of an atom
(i) In an attempt to account for the structure (splitting) of line spectra known as fine spectra, sommerfeld modified atomic model by adding elliptical orbits.
(ii) Bohr's first orbit contains only one circular orbit. Bohr's second orbit containing one circular and one elliptical orbits. Bohr's third orbit contains one circular orbit and two elliptical orbits.
(iii) It is successful in accounting for the fine line structure of hydrogen atomic spectra.
(iv) This model failed to account for the atomic spectra of atoms of more than one electron.
26) It is impossible to determine accurately and simultaneously, the position and velocity of electron in an atom. It is called Heisenberg's uncertainty principle.
27) According to Debroglie theory all moving particles have wave properties.
28) A Quantum mechanical model of atom was developed by Erwin schrodinger. According to this model of an atom, instead of orbits of Bohr's model. The electrons are thought to exist in a particular region of space around the nucleus at a given instant of time.

29) The region of space around the nucleus where the probability of finding the electron is maximum is called an orbital.
30) Principal quantum number was proposed by Neils Bohr.
31) The values of principal quantum numbers, n are 1, 2, 3, 4............. etc.
32) First orbit n = 1 or K
       Second orbit n = 2 or L.
       Third orbit n = 3 or M.
       Fourth orbit n = 4 or N.
33) Principal Quantum number indicates the size and energy of the orbit.
34) Azimuthal Quantum number was proposed by SommerFeld.
35) The Quantum number (l) is the angular momentum quantum number. 'l' has integer values from 0 to n − 1 for each value of 'n' each 'l' value represents one sub shell.
36) Each value of  pf 'l' is related to the shape of a particular sub-shell in the space around the nucleus.
37) The value pf 'l' for particular sub shell is generally designated by the letters s, p, d.... as follows

38) When n = 1, there is only one sub-shell with l = 0 This is designated as '1s'orbital.
39) When n = 2, there are two sub-shells, with l = 0, the '2s' sub shell and with l = 1, the '2p' sub-shell.
40) Magnetic quantum number was proposed by Lande.
41) The value of a m_l are −l through o to + l.
42) The total values of m_l are 2l + 1 and indicate the total number of orbitals in the sub shell.
43) The maximum number of electrons in a sub shell is equal to 2(2l + 1)
44) Magnetic quantum number indicates the orientation of orbitals in space.

47) Spin quantum number indicates the direction of the spin of the electron.
48) s − orbitals are spherical in shape.
49) p − orbitals are dumb bell in shape.

60. The electronic configuration of chronium
      (z = 24) is 1s¹ 2s² 3s² 3p⁶ 4s¹ 3d⁵

Conceptual flow chart