Atomic Structure and Periodic Trends

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Slide 81

n

l

Possible

# of e

n = 2

2p

n = 3

3s

3p

3d

l = 1 (p)

# of orbitals

2l+1

2s

Slide 82

Principles of how to build up electron configurations

The Aufbau Principle - “The building-up”principle

When establishing the ground state configuration of an atom start at the energetic bottom and work your way up

2p

3s

3p

3d

1s

2s

NB: The energy ordering of the orbitals changes

with the number of electrons.

4s

Slide 83

Principles of how to build up electron configurations

(1) The Pauli Exclusion Principle - No two electrons in one atom may have the same set of four quantum numbers (that is they must differ in one or more of n, l, ml , ms)

Slide 84

Differ in

Differ in

Differ in

Differ in

Slide 85

Remember, when two electrons share one orbital, their magnetic spin quantum numbers must be

Slide 86

Principles of how to build up electron configurations

(2) HUND’s Rule - When electrons occupy orbitals of the same energy, the lowest energy state corresponds to the configuration with the greatest number of unpaired electrons

E(Paired) E(Unpaired)

Energetically favored

Slide 87

Maximizing the number of parallel spins - The exchange interaction

Quantum mechanical in origin

Arguments based on the fact that total wavefunction has to be with respect to exchange of the electrons (Pauli)

Nothing to do with the fact that electrons are charged!

Result is that each electron pair with parallel spins leads to a lowering of the electronic energy of the atom

Slide 88

Now, let’s start remembering

The Pauli Exclusion Principle (only two anitparallel spins in one orbital)

(2) Hund’s Rule (parallel spin configuration of lower energy for degenerate orbitals)

(3) Aufbau Principle (from energetic bottom to energetic top)

Slide 89

2He

Configuration 1s2

Full Shell

Inert

Noble Gas

Unlikely to form bonds

or ions

1H

Configuration 1s1

Can lose one electron to from stable ion H+

Can form single bond

H2O,

H2

Slide 90