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2.3 Electron Arrangement
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- Light consists of electromagnetic waves
Secretly we’re in Physics!
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Electromagnetic Radiation
• Most subatomic particles behave as PARTICLES and obey the physics of waves.
wavelength Visible light
wavelength
Ultraviolet radiation
Amplitude
Node
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• Waves have a frequency• Use the Greek letter “nu”, ν, for frequency,
and units are “cycles per sec”
• All radiation: λ • ν = c!!where c = velocity of light = 3.00 x 108 m/sec
• So, as wavelength decreases, frequency increases!
Electromagnetic Radiation
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Electromagnetic SpectrumLong wavelength --> small frequencyShort wavelength --> high frequency
increasing frequency
increasing wavelength
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6Spectrum of White Light
When white light, containing all wavelengths of light hits a prism, the different wavelengths are bent so that light is broken into its componentsThis gives a CONTINUOUS SPECTRUM of colours.
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Electromagnetic Spectrum
In increasing energy, ROY G BIV
High frequency
Short wavelength
High energy
lower frequency
longer wavelength
lower energy
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Why do we care about WAVES?! This is
CHEMISTRY, not PHYSICS!
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9Atomic Line Emission
Spectra and Niels BohrBohr’s greatest contribution to science was in building a simple model of the atom.
It was based on an understanding of the LINE EMISSION SPECTRA of excited atoms.
Niels Bohr(1885-1962)
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10Line Emission Spectra of Excited Atoms
• Evidence that supported Bohr’s model came from Line Emission Spectra
• Excited atoms emit light of only certain wavelengths
• The wavelengths of emitted light depend on the element.
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Emission of Light• When electrons are
excited they can move to a higher energy level.
• When they move back down they emit energy in the form of light
• The colour of the light emitted depends on the FREQUENCY
• This light forms a LINE SPECTRUM
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What if Bohr had seen a continuous spectrum (all the
colours) instead of a line spectra?
How would the model of the atom be different?
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Line Spectra• Because electrons can only exist in
certain energy levels, only certain transitions can occur.
• e.g. an electron can move from the second energy level (n=2) to the first energy level (n=1).
• Each transition emits light of a particular frequency.
• Therefore a line spectrum is produced.• A continuous spectrum would suggest
that electrons can exist anywhere around the nucleus
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14Spectrum of Excited Hydrogen Gas
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Line Spectra of Other Elements
If we see visible light when electrons land in the second shell, what would happen when the electron lands in the first shell? Or the third shell?
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Balmer Series• Balmer determined the
wavelengths of the four lines in the VISIBLE hydrogen line spectra
• The energy of the lines corresponds to the difference in energies between outer shells and the second electron shell of hydrogen
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Similar work done by Paschen for the infrared series and Lyman for ultra violet series
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Hydrogen Emission Spectrum
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Notes• Notice that at higher frequencies the lines
get closer together (converge)• This indicates that the energy difference
between levels becomes less as the level number increases.
• In other words: the difference in energy between n=1 and n=2 is GREATER than the difference in energy between n=2 and n=3
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The lower energy part of the spectrum
• Represents lower energy transitions• E.g. an electron moving from n=4 to n=3• These energy levels are closer so less
energy is emitted.• Convergence is also observed here, e.g
lines for n=6 n=3 will be extremely close to the line for n=7 n=3
• Each series will come to an end with the line for the transition from n = ∞
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22Bohr’s Ideas
• Electrons move around the nucleus in shells or energy levels
• The electrons pathway is called an orbit
• Different energy levels contain different numbers of electrons (2n2)
• The outer electrons are called valence electrons
• The lowest energy state of an atom is called the ground state
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Electron Arrangement
• Sodium has 11 electrons• 2 electrons are contained in the first
shell (2n2)• 8 electrons are contained in the
second shell• 1 electron is contained the third shell• Therefore: 2,8,1
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