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Atomic Structure

Atomic Structure PDF Notes, Important Questions and Synopsis

SYNOPSIS

Dalton’s atomic theory:
Dalton’s atomic theory is based on the law of conservation of mass and the law of definite proportions. He also proposed the law of multiple proportions.

Thomson’s model of atom:

• It is also known as watermelon model or plum pudding model.
• According to this model, the atom is a positive sphere in which negative charges are distributed.

Rutherford alpha scattering experiment:

• This experiment was performed to understand the structure of an atom.
• This model concludes that most of the part of an atom is empty and each atom consists of a heavy positively charged nucleus.
Fundamental particles of an atom:

 Particle Mass (amu) Charge Discoverer Electron 1.0073 +1 JJ Thomson Proton 1.0087 0 Goldstein Neutron 0.0005 −1 Chadwick

Bohr’s model of atom:

• Electrons orbit the nucleus in orbits which have a set size and energy.
• The lowest energy is found in the smallest orbit.
• Radiation is absorbed or emitted.

• Wavelength: It is the distance between successive points of equal phases of a wave.
• Frequency: It is the number of cycles or oscillations or vibrations of wave motion in unit time.
• Velocity: It is the distance travelled by the wave in one second.
• Wave number: It is defined as the number of waves in unit wavelength.

Heisenberg’s uncertainty principle:
The uncertainty principle says that we cannot measure the position and momentum of the particle with absolute precision.

Bohr–Sommerfield model:

• According to this model, the path of an electron around the nucleus is an ellipse with the nucleus at one of its foci.
• The angular momentum of an electron in a closed elliptical path is also quantised.

Orbital:
An orbital is a three-dimensional region in which the probability of finding an electron is maximum.

Quantum numbers:
Principal quantum number:

• It is denoted by n.
• n = 1, 2, 3, 4 … ∞
• n = 1 K shell; n = 2 L shell
• n = 3 M shell; n = 4 N shell
• As the value of ‘n’ increases, the energy of the electron increases.

Azimuthal or angular quantum number:

• It is denoted by l.
• The values of l are from 0 to (n-1)
• l = 0 s subshell; l = 1 p subshell
• l = 2 d subshell; l = 3 f subshell
• The value of ‘l’ signifies the shape and energy level of subshells in a major energy shell.

Magnetic quantum number:

• It is denoted by ml.
• The values of ml lie from –l to +l.
• The values of ml signify the possible number of orientations of subshells.

Spin quantum number:

• It is denoted by ms.
• The values of ms are +1/2 and −1/2.
• The values of ms signify the direction of rotation or the spin of an electron in its axis during motion.
Shape of orbitals:

 Subshell Shape of orbital s Spherical p Dumbbell d Double dumbbell f Diffused shape

Pauli’s exclusion principle:

• It is impossible for two electrons of an atom to have all its four quantum numbers same.
• Maximum number of electrons in a shell can be 2n2.
• Maximum number of electrons in a subshell can be 2 only.

Aufbau principle:

• Orbitals with lower energy level are filled first.

(n+l) rule:

• The subshell with lower values of (n+l) possesses lower energy.
• If (n+l) for two orbitals are the same, the one with lower value of n possesses lower energy and should be filled first.

Paramagnetism:
A substance whose molecules, ions or atoms have unpaired electrons is paramagnetic.

Diamagnetism:
Molecules, ions or atoms having no unpaired electrons are diamagnetic.

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