Chapter_2.1_A

*atomic structure*CHAPTER 2

ATOMIC STRUCTURE2.1 Bohrs Atomic Model

2.2 Quantum Mechanical Model

2.3 Electronic Configuration

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*atomic structure*Bohrs Atomic Model At the end of this topic students should be able to:-Describe the Bohrs atomic postulates.Explain the existence of electron energy levels in an atom and calculate the energy of electronDifferentiate between line spectrum and continuous spectrum.Perform calculation involving the Rydberg equation for Lyman, Balmer, Paschen, Brackett and Pfund seriesOutline the weaknesses of Bohrs atomic model.

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*atomic structure*In 1913, a young Dutch physicist, Niels Bhr proposed a theory of atom that shook the scientific world.The atomic model he described had electrons circling a central nucleus that contains positively charged protons. Bhr also proposed that these orbits can only occur at specifically permitted levels only according to the energy levels of the electron and explain successfully the lines in the hydrogen spectrum.BOHRS ATOMIC MODELS

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*atomic structure*Electron moves in circular orbits about the nucleus. In moving in the orbit, the electron does not radiate any energy and does not absorb any energy.

PostulatesBOHRS ATOMIC MODELS

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*atomic structure*2) The energy of an electron in a hydrogen atom is quantised, that is, the electron has only a fixed set of allowed orbits, called stationary states. n=1n=2n=3H Nucleus (proton)[ orbit = stationary state = energy level = shell ]BOHRS ATOMIC MODELSPostulates

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*atomic structure*At ordinary conditions the electron is at the ground state (lowest level). If energy is supplied, electron absorbed the energy and is promoted from a lower energy level to a higher ones (excited state). (Electron is excited)

4.Electron at its excited states is unstable. It will fall back to lower energy level and released a specific amount of energy in the form of light. The energy of the photon equals to the energy difference between the levels.BOHRS ATOMIC MODELSPostulates

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*atomic structure*e-e- absorb energy, promoted to excited statee- unstable, fall back to lower energy level

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*atomic structure*Ground state the state in which the electrons have their lowest energy

Excited statethe state in which the electrons have shifted from a lower energy level to a higher energy level

Energy levelenergy associated with a specific orbit or statePoints to Remember

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*atomic structure*The energy of an electron in its level is given by:

RH (Rydberg constant) = 2.1810-18J.n (principal quantum number) = 1, 2, 3 . (integer)Note:n identifies the orbit of electronEnergy is zero if electron is located infinitely far from nucleusEnergy associated with forces of attraction are taken to be negative (thus, negative sign) THE BOHR ATOM

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*atomic structure*Radiant energy emitted when the electron moves from higher-energy state to lower-energy state is given by the difference in energy between energy levels:

THE BOHR ATOME = Ef - EiwhereThus,

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*atomic structure*The amount of energy released by the electron is called a photon of energy.A photon of energy is emitted in the form of radiation with appropriate frequency and wavelength. where; h (Plancks constant) =6.63 10-34 J s = frequency THE BOHR ATOM

E = h

Where;c (speed of light) = 3.00108 ms-1Thus,

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*atomic structure*n =1n = 2n = 3n = 4Electron is excited from lower to higher energy level. A specific amount of energy is absorbedE = h = E3-E1 (+ve)Electron falls from higher to lower energy level .A photon of energy is released.E = h = E1-E3 (-ve)

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*atomic structure*Energy level diagram for the hydrogen atomPotential energyn = 1n = 2n = 3n = 4n = Energy releasedEnergy absorbed

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*atomic structure**Example Calculate:i) The E of an e- has when it occupies when it was at n=3 & n=4.ii) The E of the photon emitted when one mole e- of drops from the fourth E level to third E level.iii) The frequency & wavelength of this photon.*

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*atomic structure**Solution :i) at n=3, E3 = -RH 1 = - 2.18 x 10-18 J 32 9 = -2.422 x 10-19 J at n=4, E4 = -RH 1 = - 2.18 x 10-18 J 42 16 = -1.363 x 10-19J*

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*atomic structure**n = 4n = 3E = Ef Ei = E3 E4 = -2.422x 10-19J- (-1.363 x 10-19J) = -1.06 x 10-19J -ve sign indicates that E is released when e- falls.ii)E released by 1 mol of e-,E = -1.06 x 10-19J x 6.023 x 1023 mol-1 = - 63 843.8 Jmol-1 = -63.8438 kJmol-1

E*

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*atomic structure**iii) Frequency, v = E = 1.06 x 10-19J h 6.63 x 10-34 Js = 1.599 x 1014 s-1 = 1.599 x 1014Hz# -ve sign of E is ignored because frequency is always +ve

Wavelength, = c = 3.0 x 108 ms-1 v 1.599 x 1014 s-1 = 1.876 x 10-6 m = 1876 nm # 1 m = 109 nm *

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*atomic structure*Exercises:Calculate the energy of an electron in the second energy level of a hydrogen atom. (-5.448 x 10-19 J)

Calculate the energy of an electron in the energy level n = 6 of a hydrogen atom.

3) Calculate the energy change (J), that occurs when an electron falls from n = 5 to n = 3 energy level in a hydrogen atom. (answer: 1.55 x 10-19J)

Calculate the frequency and wavelength (nm) of the radiation emitted in question 3.

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