Bohr Model and Atomic Spectra

8/3/2019 Bohr Model and Atomic Spectra

1/18

Quantum Mechanics

& Spectroscopy

Atomic

Spectra

Absor

ption

Spectr

a

Emi

ssio

n

spe

ctra

Atomic

Structure

Unused

Sectio

n

Space

1

Rutherford

Model

J J

Thomson

Model


2/18

The Smallest Indivisible Part Of

A Substance Is Called As

AN ATOM


3/18

Structure of the Atom

Evidence in 1900 indicated thatthe atom was not a fundamental unit:

1) There seemed to be too many kinds

of atoms, each belonging to a distinct chemicalelement (way more than earth, air, water, and

fire!)

2) Elements combine with some elements but not

with others, a characteristic that hinted at aninternal atomic structure.

3) The discoveries of radioactivity, x rays, and the

electron all seemed to involve atoms breaking

apart in some way


4/18

Electrons (discovered in

1897) carried the negative

charge.

Electrons were very light,

even compared to the atom.

Protons had not yet been

discovered, but clearly

positive charge had to be

present to achieve chargeneutrality.


5/18

Thomsons plum-pudding model of the atom had thepositive charge spread uniformly throughout a sphere the

size of the atom, with electrons embedded in the uniform

background.

In Thomsons view, when the atom was heated, the electrons

could vibrate about their equilibrium positions, thus producing

electromagnetic radiation.

Unfortunately, Thomson couldnt explain spectra with thismodel.


6/18


7/18

Experiments of Geiger and Marsden

Rutherford, Geiger, and Marsdenconceived a new technique forinvestigating the structure ofmatter by scattering a particlesfrom atoms.


8/18

Rutherford Explanation & Model

Atom is composed ofTiny nucleus in which its

positive charge and

nearly all mass was

concentrated, with

electrons placed some

distance away.


9/18

R

Fc = Fe


10/18

Rutherford Scattering Formula

N() = Ni n t Z 2e482 r2KE2sin4(/2)Gold Nucleus Radius= 3.0 x 10-14 m


11/18


12/18


13/18

Splitting of Wavelengths


14/18

Absorption Spectrum

When white light is passed through a

a gas ,the gas is found to absorb light

of certain wavelength .So we get a

bright spectrum with dark lines on it.This is known as Absorption

spectrum.


15/18

It is observed that for every line in atomic absorption

spectrum of an element , there is a line in emission

spectrum. However reverse is not true.


16/18

Emission Spectrum

When an atomic gas or vapor at small

pressure is suitably excited , the

emitted radiation has a spectrum

which contains certain specificwavelengths only. Such a spectrum is

known as Emission Spectrum.


17/18

Hydrogen spectrum

Lyman UV R(1/12-1/n2)

Balmer Visible R(1/22-1/n2)

Paschen IR R(1/32-1/n2)

Brackett IR R(1/42-1/n2)

Pfund IR R(1/52-1/n2)


18/18

Bohrs postulates

1)An electron moves in a circular orbit around the nucleus underinfluence of columbic force.

2) It is only possible for electron to move in an orbit for which its

orbital angular momentum L is integral multiple of h/2 .L=nh/2

3)Despite the fact that electron is constantly accelarating,it will not

radiate.

4)EM wave will be radiated when electron jumps from one orbit to

other.

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Bohr Model and Atomic Spectra