The Bohr model for the electronsThe Bohr model for the electrons

Electronic structure – how the electrons Electronic structure – how the electrons are arranged inside the atomare arranged inside the atomApplying the quantum principle of energyApplying the quantum principle of energyTwo parameters:Two parameters:– EnergyEnergy– PositionPosition

Learning objectivesLearning objectives

Describe the basic principles of the Bohr modelDescribe the basic principles of the Bohr modelDistinguish between the “classical” view and the Distinguish between the “classical” view and the “quantum” view of matter“quantum” view of matterDefine atomic orbitalsDefine atomic orbitalsDistinguish between the Bohr orbit and atomic Distinguish between the Bohr orbit and atomic orbitalorbitalApply quantum numbers and atomic orbitals to Apply quantum numbers and atomic orbitals to building atoms and the periodic tablebuilding atoms and the periodic tableDescribe periodic trends in terms of electronic Describe periodic trends in terms of electronic structurestructure

Bohr’s theory of the atom: applying Bohr’s theory of the atom: applying photons to electronic structurephotons to electronic structure

Electrons occupy specific Electrons occupy specific levels (orbits) and no levels (orbits) and no othersothersOrbits have Orbits have energyenergy and and sizesizeElectron excited to higher Electron excited to higher level by absorbing photon level by absorbing photon Electron relaxes to lower Electron relaxes to lower level by emitting photonlevel by emitting photonPhoton energy exactly Photon energy exactly equals gap between levelsequals gap between levelsLarger orbits are at higher Larger orbits are at higher energyenergy

Size of energy gap determines Size of energy gap determines photon energyphoton energy

Small energy gap, low Small energy gap, low frequency, long frequency, long wavelength (red shift)wavelength (red shift)

High energy gap, high High energy gap, high frequency, short frequency, short wavelength (blue shift)wavelength (blue shift)

The full spectrum of lines for HThe full spectrum of lines for H

Each set of lines in the H spectrum comes from Each set of lines in the H spectrum comes from transitions from all the higher levels to a particular level.transitions from all the higher levels to a particular level.The lines in the visible are transitions to the second levelThe lines in the visible are transitions to the second level

The Bohr orbitsThe Bohr orbits

Bohr orbits have quantum numbers Bohr orbits have quantum numbers nn– n = 1 (capacity 2)n = 1 (capacity 2)– n = 2 (capacity 8)n = 2 (capacity 8)– n = 3 (capacity 8)n = 3 (capacity 8)

Bohr orbits and the periodic tableBohr orbits and the periodic table

Elements in the same group have the Elements in the same group have the same number of electrons in outer Bohr same number of electrons in outer Bohr orbitorbit

Successes and shortcomings of BohrSuccesses and shortcomings of Bohr

Couldn’t explain why orbits were allowedCouldn’t explain why orbits were allowedOnly successful agreement with experiment was Only successful agreement with experiment was with the H atomwith the H atom

Introduced connection between spectra and Introduced connection between spectra and electron structureelectron structureConcept of allowed orbits is developed further Concept of allowed orbits is developed further with new knowledgewith new knowledgeNonetheless, an important contribution, worthy Nonetheless, an important contribution, worthy of the Nobel prizeof the Nobel prize

Electrons are waves too!Electrons are waves too!

Life at the electron level is very differentLife at the electron level is very differentKey to unlocking the low door to the secret garden of Key to unlocking the low door to the secret garden of the atom lay in accepting the wave properties of the atom lay in accepting the wave properties of electronselectronsDe Broglie wave-particle duality De Broglie wave-particle duality All particles have a wavelength – wavelike nature. All particles have a wavelength – wavelike nature. – Significant only for very small particles – like electrons or Significant only for very small particles – like electrons or

photons photons – As mass increases, wavelength decreasesAs mass increases, wavelength decreases

Electrons have wavelengths about the size of an atom Electrons have wavelengths about the size of an atom – Electrons are used for studying matter – electron microscopyElectrons are used for studying matter – electron microscopy

Electron microscopes can peer within – Electron microscopes can peer within – waves interacting with matterwaves interacting with matter

Heisenberg Uncertainty Principle: Heisenberg Uncertainty Principle: the illusive electronthe illusive electron

We can predict the motion of a ball;We can predict the motion of a ball;But not an electron: problems locating small But not an electron: problems locating small objectsobjects

The Quantum Mechanics: waves of The Quantum Mechanics: waves of uncertaintyuncertainty

System developed that incorporated these System developed that incorporated these concepts and produced an concepts and produced an orbitalorbital picture picture of the electronsof the electronsNo longer think of electrons as particles No longer think of electrons as particles with precise location, but as waves which with precise location, but as waves which have probability of being in some region of have probability of being in some region of the atom – the orbitalthe atom – the orbitalImpossible with the Impossible with the classicalclassical mechanics of mechanics of NewtonNewton

Orbitals are described by quantum Orbitals are described by quantum numbersnumbers

Each orbital has unique setEach orbital has unique set

1s, 2p, 3d etc.1s, 2p, 3d etc.

Number describes energyNumber describes energy

Letter describes shapeLetter describes shape– S zero dimensions S zero dimensions – P one dimensionP one dimension– D two dimensionsD two dimensions– F three dimensionsF three dimensions

Getting from the orbitals to the Getting from the orbitals to the elementselements

All elements have the same setAll elements have the same set

Atomic number dictates how many are Atomic number dictates how many are filled – how many electrons are addedfilled – how many electrons are added

Filling orbitals follows a fixed pattern: Filling orbitals follows a fixed pattern: lowest energy ones firstlowest energy ones first

Orbital energy levels in H and other Orbital energy levels in H and other elementselements

How many per orbital?How many per orbital?

Electrons share orbitals (only two allowed)Electrons share orbitals (only two allowed)

A consequence of “spin”A consequence of “spin”

How many electrons can be added How many electrons can be added to the orbitalsto the orbitals

1s, 2s, 3s etc. 1s, 2s, 3s etc. 2 electrons 2 electrons

2p, 3p, 4p etc.2p, 3p, 4p etc. 6 electrons 6 electrons

3d, 4d etc. 3d, 4d etc. 10 electrons10 electrons

4f, 5f etc. 4f, 5f etc. 14 electrons14 electrons

Add electrons to the orbitals – Add electrons to the orbitals – lowest firstlowest first

2p

3d

3p

4p4s

3s

2s

1s

H(z = 1)

2p

3d

3p

4p4s

3s

2s

1s

He(z = 2)

Fill lowest orbitalFill lowest orbital

2p

3d

3p

4p4s

3s

2s

1s

Li(z = 3)

Begin next orbitalBegin next orbital

2p

3d

3p

4p4s

3s

2s

1s

Be(z = 4)

Fill 2sFill 2s

2p

3d

3p

4p4s

3s

2s

1s

B(z = 5)

Begin filling 2pBegin filling 2p

2p

3d

3p

4p4s

3s

2s

1s

C(z = 6)

Electrons don’t like to pairElectrons don’t like to pair

2p

3d

3p

4p4s

3s

2s

1s

O(z = 8)

2p

3d

3p

4p4s

3s

2s

1s

F(z = 9)

2p

3d

3p

4p4s

3s

2s

1s

Ne(z = 10)

Filled 2p – neon unreactiveFilled 2p – neon unreactive

Shape of the periodic table Shape of the periodic table explained by orbital pictureexplained by orbital picture

2 groups 10

groups

14 groups

6 groups

Shells: echoes of the Bohr orbitsShells: echoes of the Bohr orbits

The orbitals with the same Principal The orbitals with the same Principal Quantum number (1,2,3 etc) are grouped Quantum number (1,2,3 etc) are grouped into shellsinto shells

Filled shells have special significanceFilled shells have special significance

The periodic lawThe periodic law

Ionization energy and the periodic lawIonization energy and the periodic law

Ionization energy is energy required to remove electron Ionization energy is energy required to remove electron from the neutral atomfrom the neutral atom