back ©bires, 2002 slide 1 bires, 2009 chapter 4 electron configurations and quantum chemistry...

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©Bires, 2002 Slide 1Bires, 2009

Chapter 4Electron Configurations and

Quantum Chemistry

Electron configurations determine how an atom behaves in bonding

with other atoms!Topics rearranged from your text, pages 90-116.

Atomic Emissions/Abortions removed

Anyone who says that they can contemplate quantum mechanics without becoming dizzy has not understood the concept in the least. -Niels Bohr

electron

neutron

proton

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The Bohr Model• Niels Bohr

– rebuilt the model of the atom placing the electrons in energy levels.

• Quantum chemistry– a discipline that states that energy can be given off

in small packets or quanta of specific size.

• What would happen to an electron if the right sized quanta of energy was added to it?

• What would happen when the electron came back down to its ground state?

EXCITED STATE

Ground state

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Electron Configurations - overview• Bohr model

– electrons exist in specific energy levels.

• Electron orbitals (shapes)– Within each energy level, the orbits the electrons

can occupy.

• Within each orbital– electrons can be set “spin up” or “spin down”

• Electron configuration– The configuration of electrons in their levels,

orbitals, and spins.

• Modern Quantum Model– Electron exists in electron configurations

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Energy Levels (n)• The electrons exist in energy

levels or shells.

• The first energy shell can hold only 2 electrons.– Hydrogen and Helium in their

ground state have electrons that occupy this shell.

• The second shell can hold 8 electrons.

• The third can hold 18 electrons.

2 8

32

18

Shells

All shells after three can hold 32 electrons.

Old School: “KLM notation”

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Orbitals (Shapes)• Orbitals

– electrons travel in set paths.– These paths form shapes, called

orbitals.

• Each “shape” can hold 2 electrons

• The smallest orbital is the “s” orbital. The “s” orbital:– Has only 1 shape (holds 2 e-)– Is spherical in shape– Is the lowest energy orbital s-2

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p-Orbitals• The 2nd orbital shape is the “p” orbital shape.• There are 3 “p” shapes, each holding 2

electrons, for a total of 6 electrons in the “p” orbitals.

• The “p” orbitals are:– Dumbbell-shaped

– Higher in energy than the “s” p-6s-2

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d-Orbitals• The 3rd orbital shape is the “d” orbital shape.

• There are 5 “d” orbital shapes, for a total of 10 electrons in the “d” orbitals.

• “d” orbitals are higher in energy than “p” orbitals.

s-2

d-10p-6

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f-Orbitals• The last orbital

shape is the “ f ” orbital shape.– “ f ” orbitals have

irregular shapes due to quantum tunneling.

– There are 7 “ f ” shapes, for a total of 14 electrons.

Electrons in f orbitals are very high in energy

s-2 f-14d-10p-6

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“Blocks” of the periodic table…• The periodic table tells us in which orbital the

last electron should be found.– The last electron in an atom is found in the…

s orbitals p orbitals

d orbitals

f orbitals

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Electron “Spin”• Electrons can be “spin up” or “spin down.”

– (by convention, an electron that is alone is “spin up”)

• Hund’s Rule– As electrons fill orbitals, they first fill each shape

available with one electron before spin pairing.

• Pauli’s Exclusion Principle– If two electrons share a shape, they must be spin-

paired (one up and one down).

• For instance: take a “p” orbital…it has three orbital-shapes that can hold 2 e- each.

• It would fill like this:Electron Configurations.mov

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Writing Electron Configurations• The The Aufbau principleAufbau principle

– electron will fill lower energy electron will fill lower energy orbitals first.orbitals first.

• Energy of electrons:– low energy s < p < d < f high energy– low energy nearer < farther high energy– low energy level 1 < level 7 high energy

• Total energy of an electron:– Product of energy of its shell and the

energy of its orbital.– Guess: Which is lower in energy, an Guess: Which is lower in energy, an

electron found in 3d or one found in 4s?electron found in 3d or one found in 4s?

s low energyd high energy

close low energy

far high energy

Total energy

=

Shell

x

orbital shape

The 4s electrons are lower in energy!

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Writing Electron Configurations• Orbital filling diagram

– Shows how electrons fill into levels and orbitals

1s Electron 2s 2p Configurations 3s 3p 3d 4s 4p 4d 4f 5s 5p 5d 5f 6s 6p 6d 6f 7s 7p 7d 7f

1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f14 6d10 7p6 6f14 Don’t Copy this

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Building the Orbital Filling Diagram• Begin by listing the shells 1, 2, 3,

4, 5, 6, 7 vertically.• These are your “s” orbitals.• Next, add another column of

number, beginning with 2.• These are your “p” orbitals.• Do the same for “d” and “f”

orbitals, beginning with “3” for the “d” orbitals and “4” for the “f” orbitals.

• Next, add your orbital letters.• Finally, draw diagonal lines as

shown.

1

2

3

65

4

7

2

3

65

4

7

3

65

4

7

65

4

7s

s

s

s

s

s

s

p

p

p

p

p

p

d

d

d

d

d

f

f

f

f

s p d f

...7654654543433221 26101426102610262622 spdfspdspdspspss

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Electron Configurations of Some Atoms• Consider Fluorine, with 9 electrons

• What about Copper, with 29 electrons?

Notice the position of the last electron…

522 221F pss

9262622 3433221Cu dspspss

2962622 4333221Cu sdpspss

Both used

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Noble Gas Shorthand• Notice the configurations of the noble gases:

• We can shorten the electron configuration of larger elements with NGS.

• Consider Mg:• We can substitute Neon’s e- config, and write Mg:• Similarly, Titanium’s (Ti) e- config:

• Can be shortened to:

21He s 622 221Ne pss 62622 33221Ar pspss

2622 3221Mg spss

23]Ne[Mg s2262622 3433221Ti dspspss

2234]Ar[Ti ds

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Ion e- configurations• Ions (elements with more/less electrons) also have

electron configurations.

• Consider Sulfur (S):

• What if sulfur gained two electrons?

• Consider Calcium (Ca):

• What if calcium lost two electrons?

4233]Ne[S ps

6233]Ne[-S2 ps

262622 433221Ca spspss

626222 33221Ca pspss

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Octets!• Octets:

– Atoms with filled s and p orbitals in the same, highest level.

– Have noble gas-like configurations– Have special stability

• Both atoms and ions can have complete octets.622 221Ne pss

62622 33221Ar pspss

6233]Ne[-S2 ps626222 33221Ca pspss

...... 62 ps

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• Question:– Why do the atomic radii (size) of atoms decrease as

electrons and protons are added to the atom, as you move from left to right across a period?

• electrostatic attraction– attraction between the electrons (-) in the shells and

the protons(+) in the nucleus – pulls the electrons in

This is what we call a periodic trend

End of chapter 4

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