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EXPLAINING THE

FORMATION OF SPECTRA THE FINGERPRINTS OF

THE ELEMENTS

The work of Niels Bohr

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Bohrs model of the atom:

Within an atom there exist certain orbits inwhich electrons can move.

An electron may not occupy any orbits

between these allowed orbits.

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Electrons in orbits further from the nucleus

have more energy than those electrons

closer to the nucleus. Each orbit can be

viewed as an energy level.

When an electron in an orbit further from

the nucleus (energy E2 ) moves to an orbit

closer to the nucleus (energy E2), a certain

amount of energy is emitted.

The levels were given the symbol n (called

the principal quantum number).

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Energyof in

different

levels

Diagram showing the energy levels in

an atom

E1

n = 1

n = 2

n = 4

n = 6

n = 3

n = 5

E2

E = E1 -E2

energy

Energy

levels

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Energy values for the electron are

negative.

When an electron is so far from the

nucleus that the force of attraction is zero,

its potential energy is zero. But, at this

distance it has the greatest amount of

potential energy. As it gets closer to the

nucleus, its potential energy decreases sothe values are negative.

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Energy levels and the line emission

spectra

When the electron in a hydrogen atom isin the orbit closest to the nucleus, it

possesses the least amount of energy.

This is called the ground state. If the electron receives energy it can

jump to a higher energy level. This

energy can come from the high voltage ofa discharge tube or the high temperature

of a bunsen flame.

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The wavelength of this emitted radiation

is:

= h c

E1 E2

A big energy drop results in the emission

of electromagnetic radiation of short

This amount of energy (E) is associated with

light of specific frequency, according to the

equation E = hf (Plancks equation). f = c and so E = h c

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Paschen

series

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Energy levels and the line absorption

spectra

When light is shone through a cool gas,electrons absorb only those photons of light(quanta) that cause them to rise to allowedenergy levels within the atom.

The energy values of these photons correspondto the energy values of the photons in theemission spectrum.

Analysis of the absorption spectra of distantstars allows astronomers to predict whatgaseous elements surround the star.

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Does Bohrs model always work?

Bohrs model explained the simplehydrogen atom adequately. It could not,however, explain the spectra of morecomplicated atoms.

Later models included the wave nature ofelectrons and regions in space where

electrons were able to move, rather thanorbits. These regions are called orbitals.Transitions between these sub-levels ofenergy better explained spectra.

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He won the

Nobel Prize

for Physics in

1922

He founded the

Copenhagen

Institute for

Theoretical Physics

at the age of 35.

He based his model on

the discovery that

electromagneticradiation is quantized

and the energy of the

photon can be

calculated using

Plancks equation.

He was born in

Denmark and

later moved to

Cambridge to

do research.