the bohr model for the electrons - college of dupage - … quantum numbers and atomic orbitals to...
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The Bohr model for the electrons
Electronic structure – how the electrons are arranged inside the atom
Applying the quantum principle of energy
Two parameters: – Energy
– Position
Learning objectives
Describe the basic principles of the Bohr model
Distinguish between the “classical” view and the “quantum” view of matter
Define atomic orbitals
Distinguish between the Bohr orbit and atomic orbital
Apply quantum numbers and atomic orbitals to building atoms and the periodic table
Describe periodic trends in terms of electronic structure
Bohr’s theory of the atom: applying
photons to electronic structure Electrons occupy specific levels (orbits) and no others
Orbits have energy and size
Larger orbits are at higher energy
Electron excited to higher level by absorbing photon
Electron relaxes to lower level by emitting photon
Photon energy exactly equals gap between levels
Size of energy gap determines
photon energy
Small energy gap, low
frequency, long
wavelength (red shift)
High energy gap, high
frequency, short
wavelength (blue shift)
The full spectrum of lines for H
Each set of lines in the H spectrum comes from transitions from all the higher levels to a particular level.
The lines in the visible are transitions to the second level
The Bohr orbits
Bohr orbits have quantum numbers n
– n = 1 (capacity 2)
– n = 2 (capacity 8)
– n = 3 (capacity 8)
Bohr orbits and the periodic table
Elements in the same group have the same
number of electrons in outer Bohr orbit
Successes and shortcomings of Bohr
Couldn’t explain why orbits were allowed
Only successful agreement with experiment was with the H atom
Introduced connection between spectra and electron structure
Concept of allowed orbits is developed further with new knowledge
Nonetheless, an important contribution, worthy of the Nobel prize
Electrons are waves too!
Life at the electron level is very different
Key to unlocking the low door to the secret garden of the atom lay in accepting the wave properties of electrons
De Broglie wave-particle duality
All particles have a wavelength – wavelike nature. – Significant only for very small particles – like electrons or
photons
– As mass increases, wavelength decreases
Electrons have wavelengths about the size of an atom – Electrons are used for studying matter – electron microscopy
Heisenberg Uncertainty Principle:
the illusive electron We can predict the motion of a ball;
But not an electron: problems locating small objects
The Quantum Mechanics: waves of
uncertainty
System developed that incorporated these concepts and produced an orbital picture of the electrons
No longer think of electrons as particles with precise location, but as waves which have probability of being in some region of the atom – the orbital
Impossible with the classical mechanics of Newton
Orbits become orbitals
The Orbitron: a gallery
of atomic orbitals and
molecular orbitals
Orbitals are described by quantum
numbers
Each orbital has unique set
1s, 2p, 3d etc.
Number describes energy
Letter describes shape
– S zero dimensions
– P one dimension
– D two dimensions
– F three dimensions
Getting from the orbitals to the
elements
All elements have the same set
Atomic number dictates how many are
filled – how many electrons are added
Filling orbitals follows a fixed pattern:
lowest energy ones first
How many electrons can be added
to the orbitals
1s, 2s, 3s etc. 2 electrons
2p, 3p, 4p etc. 6 electrons
3d, 4d etc. 10 electrons
4f, 5f etc. 14 electrons
Simplifying with shells:
echoes of Bohr orbits The orbitals with the same Principal Quantum
number (1,2,3 etc) are grouped into shells
Filled shells have special significance
Unfilled
shell 2 Filled
shell 2
Filled
shell 1 Filled
shell 1
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