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ENGINEERING CHEMISTRY, Module I, , Unit I, , Atomic Structure, Bohr Model of atom – Postulates, Merits and Demerits - Dual nature of matter – de Broglie, relation – Uncertainty Principle – Concept of Orbit and Orbital – Quantum numbers – Sub, energy levels (s,p,d,f) - shape of s and p orbitals., Electronic Configuration of atom – Aufbau principle, Pauli’s exclusion principle, Hund’s rule of, maximum multiplicity – electronic configuration of elements up to atomic number 20.

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Bohr Model, Postulates of Bohr Model, , Niels Bohr was a danish, physicist who made, foundational, contributions, to, understanding atomic, structure and quantum, theory, for which he, received the Nobel, Prize in Physics in, 1922., , 1. Electrons in an atom are revolving around the nucleus in, fixed circular paths called orbits or shells., The orbits are designated as K,L,M,N etc., or numbered as 1,2, 3,4 etc., 2. Allowed orbits are those for which angular momentum is an integral, multiple of h/2π., mvr = 2π, Thus angular momentum and hence energy of the electron is quantised., The angular momentum corresponding to different orbits are, K – shell, M – shell, , n=1, n=3, , mvr =, mvr =, , 2π, 2π, , L – shell, and so on., , n=2, , mvr =, , π

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Bohr Model, Postulates of Bohr Model, 3. The centrifugal force of the revolving electron is balanced by, electrostatic force of attraction., m, r, , = 4π, , 4. As long as the electron remains in the given orbit its energy does not, change., , Emission, , 5. The energy changes when electron jumps from one orbit to another ,, ie, during the transitions., When the electron jumps from higher orbit to lower orbit, the process is, known as emission and when it jumps from lower orbit to a higher one, the process is absorption., Absorption

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Bohr Model, Merits of Bohr Model, , 1.Bohr model could explain stability of atom in terms of stationary states., 2. It helped to calculate radius of orbit, velocity and energy of the electron., 3. It is able to explain atomic spectra of Hydrogen., Demerits of Bohr Model, 1.Bohr model could not explain fine structure of atomic spectra., , 2. It is unable to explain splitting of spectral lines in electric field (Stark effect) and in, magnetic field (Zeeman effect) ., 3. It does not mention anything about the formation of molecules by chemical bonding.

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Atomic structure, , Modern atomic theory, , Modern atomic theory is based on, , 1, 2, 3, , Dual nature of electrons, , Uncertainty in position and, momentum of electrons, , Quantisation of energy level

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Atomic structure, de Broglie relation, According to de Broglie, microscopic particles like electrons exhibit, wave nature along with particle nature(dual nature)., The wavelength of a microscopic particle is known as, de Broglie wavelength(λ) which is given as, , λ=, h - Plank’s constant, m - mass of the particle, v - velocity of the particle, , de Broglie was a French, physicist, who, made, groundbreaking contributions, to quantum theory. In his 1924, PhD thesis, he postulated the, wave nature of electrons and, suggested that all matter has, wave properties.

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Atomic structure, Heisenberg’s Uncertainty Principle, According to uncertainty principle, it is impossible to determine, simultaneously the position and momentum of a microscopic particle, exactly., The principle is mathematically expressed as, , Δx Δp, Δx and Δp are the uncertainty in position and momentum of the electron, h - Plank’s constant, , Werner Heisenberg was a, German theoretical physicist, and one of the key pioneers of, quantum mechanics.

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Atomic structure, Orbital, According to uncertainty principle , the position of a moving, electron can not be determined exactly. Therefor the fixed, orbit of Bohr model is replaced by orbital., Orbital is the region of space around the nucleus where there, is a maximum probability of finding the electron., Each orbital correspond to a definite energy level and can, accommodate only two electrons., , Plots showing the probability of, finding electrons around a, nucleus.

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Atomic structure, Quantum numbers, 1. Principal quantum number (n) – It represents shell. It can have integral values 1,2,3 …, 2. Subsidiary quantum number (l) – It represents subshell. It can have integral values 0 to (n-1)., , `, , l = 0 corresponds to s- subshell, l = 2 corresponds to d- subshell, , l = 1 corresponds to p- subshell, l = 3 corresponds to f - subshell, , 3. Magnetic quantum number (m) – It represents orbital. It can have values – l ,… 0 , ... +l., For s - subshell, For p - subshell, , m = 0 correspond to s- orbital, m = -1, 0, +1 correspond to px, py and pz orbitals, , 4. Spin quantum number (s) – It represents spin of electron. It can have only two values - 1/2 and -1/2, corresponding to clockwise and anticlockwise spin.

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Atomic structure, shape of s and p orbitals, , s orbital is spherical in shape, , p orbitals are dumb-bell in shape

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Review Questions, 1. Using Bohr model describe the structure of Lithium atom., 2. Discuss the merits and demerits of Bohr model., 3. What are the basic concepts involved in modern theory of atom., 4. How does quantum numbers used to denote the structure of atom?, 5. List the orbitals present in the second shell., 6. Give the relation showing dual nature of microscopic particles., 7. The position and momentum of a moving particle can not be determined simultaneously and precisely., Give the mathematical expression representing this statement.

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Atomic structure – Electronic configuration, Electronic configuration is the arrangement of electrons in the orbitals of an atom, The arrangement of electrons in various orbitals are governed by the following laws, 1. Aufbau Principle, 2. Pauli’s Exclusion Principle, 3. Hund’s rule of maximum multiplicity

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Electronic Configuration of atom, Pauli’s exclusion principle, No two electrons in an atom can have the same set of all the four quantum numbers., For a given orbital, first three quantum numbers are the same. The fourth spin quantum number, has only two allowed values +1/2 and -1/2. Therefore only two electrons can be accommodated in, an orbital., Illustration taking 3s orbital as an example, 3s2, 3s3, n, l, m, s, , 3, , 3, , 0, 0, , 0, 0, , +1/2, , -1/2, , 3, 0, , 3, 0, , 0, , 0, , 3, 0, 0, , +1/2 -1/2 +1/2

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Electronic Configuration of atom, Hund’s rule of maximum multiplicity, For degenerate orbitals, the pairing of electrons takes place only when each of the degenerate, orbital has at least one electron., Degenerate orbitals are orbitals having the same energy like 2px, 2py and 2pz., , Correct arrangement, , Incorrect arrangement

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Review Questions, 1., , Arrange the orbitals in the first three shells in the order of energy., , 2. ‘ No two electrons in an atom can have the same set of all the four quantum numbers’., What is the consequence of this statement on the arrangement of electrons in orbitals in, an atom?, 3. Allot four electrons in five degenerate orbitals represented as boxes shown below., , 4. Write the electronic configuration of an atom with atomic number 19.