ATOMIC SPECTRA
OBJECTIVES
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Do calculations based on Planck’s constant
explain how data from emission spectra provide evidence for
discrete energy levels within the atom;
Bohr model, the emission spectrum of hydrogen; Lyman series,
Balmer series; ΔE or dE = hν.
• What is electromagnetic radiation?
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Evidence for arrangement of electrons comes from atomic
spectra.
Atomic spectrum is formed when electromagnetic radiation(ER)
is absorbed/emitted by an element.
ELECTROMAGNETIC SPECTRUM
EM radiation behaves like a wave
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The number of waves that pass through a given point is the wave’s
frequency.
The longer the wavelength, the shorter the frequency.
Wavelength (λ) and frequency(ν) are inversely related.
EM is viewed as a stream of photons. Each photon contains a certain
amount of energy which is related to Planck’s eqn.
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ALL types of EM radiation travel at the same speed of light, c= 3.00*10^8
m/s
Wavelength and frequency can be related through Planck’s equation.
Planck’s constant links the amount of energy a photon carries with the
frequency of its EM wave.
Planck’s equation indicates energy is directly proportional to frequency.
Planck’s eqn: E=hν
E- energy
ν- frequency
h- Planck’s
constant=
6.63*10^-34Js
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Since frequency and wavelength of electromagnetic radiation is related
by
• c= νλ
ν=c/λ
E=hc/λ
A quantum of energy absorbed/emitted by an atom can be determined by
measuring ν or λ of EM radiation absorbed/emitted.
Questions
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What is the frequency of radiation that has a wavelength of
20um?
What is the wavelength of radiation that has a frequency of
6.20*1012s-1?
• Find the energy of a wave with frequency of 2*108 Hz
• Find the energy of a wave with wavelength 3*10-12m.
• Orange light has a wavelength of 620 nm.
(a) What is its wavelength in metres?
(b)What is its frequency?
(c) Now use Plank’s Law to work out the energy of one quantum of
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orange light.
Calculate the energies of one photon of light of ultraviolet (λ = 2 x
10-8 m).
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The whole range of frequencies of EM radiation is called the
electromagnetic spectrum.
Absorption/emission of energy by an element corresponds to
specific points on the EM spectrum which may be used to identify
the element.
Formation of a continuous
spectrum
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A beam of white light is passed through a prism on to a screen, a
spectrum of colours made up of all wavelengths of visible light is
seen like a rainbow.
This is called a continuous spectrum.
Continuous spectrum
Discontinuous spectrum
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If white light passes through a substance, the atoms can absorb
light of certain wavelength and dark lines appear in the spectrum.
A line spectrum is formed which appears as distinct lines and not
bands of colours. This is called a discontinuous spectrum.
Absorption spectrum
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The wavelengths of dark lines in spectrum are wavelengths of light
absorbed by the atoms. This line spectrum is an absorption spectrum.
the absorption of energy by an element corresponds to specific
wavelength on the electromagnetic spectrum and produces a unique
spectrum.
Emission spectrum
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If atoms are supplied with heat/electrical energy; they emit energies of
certain wavelengths as the excited atoms return to stable orbit. This
represents an emission spectrum. An emission spectrum is produced
which shows coloured lines on a dark background.
The emission of energy by an element corresponds to specific
wavelengths on the electromagnetic spectrum and produces a unique
spectrum.
Hydrogen emission spectrum
• Emission spectrum of hydrogen show that electrons in an atom can exist
only in discrete levels,
• According to Bohr, when energy added to electrons in ground state, they
absorb quantum of energy which causes them to move to an orbit with
higher energy level.
• This excited electron cannot maintain this position for a long time and
falls back to lower energy level.
• As it falls back it emits a quantum of energy equivalent to the difference
in energy levels. The greater the difference in energy levels, the higher
the frequency of light emitted.
Principal quantum numbers
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Bohr assigned the energy levels of the electron with the letter ‘n’, value of
n being 1 for the lowest level, 2 for the next level and so on. He named it
the principal quantum number.
Electron transition
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Emission spectrum of hydrogen consists of several series of lines.
These lines arise from the transition of electrons from orbit of
higher quantum # to orbits of lower quantum number.
Electron transition
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In each series, lines become closer together as frequency
increases until it becomes continuous spectrum. The convergence
line corresponds to the transition from an energy level where the
electron is lost from an atom and atom is ionized.
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The series of lines in visible region of the hydrogen spectrum arise
from transitions from orbits with higher energy level to energy
level n=2. this is called the Balmer series.
Electron transitions from higher energy level to energy level n=1
result in lines in the uv region of spectrum and is called Lyman
series.
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The Paschen series arise from transitions from higher energy
levels to energy level n=3.
The Brackett series arise from transitions from higher energy
levels to energy level n=4.
The Pfund series arise from transitions from higher energy levels
to energy level n=5.
Questions
• The line emission spectrum of hydrogen consists of a series of
lines in different parts of the electromagnetic spectrum.
a. Describe the processes that occur in the hydrogen atom that
results in the formation of an emission spectrum.
b. Explain how the data from the emission spectrum provide
evidence for discrete energy levels within the atom.