Electron Configurations4.3

Pauli Exclusion PrincipleNo more than two e- can occupy a single orbital at a time

e- spin in opposite directionsSpin quantum number can be +½ or -½

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Electron ConfigurationA description of the electron orbitals in an atom

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e- occupy lower energy levels firstAufbau principle – electrons in an atom will occupy the lowest energy orbitals available.

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Electron ConfigurationsRemember, the smaller the principal quantum number, the lower the energy, and the smaller the l quantum number, the lower the energy

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Electron ConfigurationsThe order in which energy levels fill is …

1s<2s<2p<3s<3p

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Electron ConfigurationsAfter this, the energy levels are less straightforward

The E levels of the 3d orbitals are slightly higher than those of the 4s orbitals.

1s<2s<2p<3s<3p<4s≈3d

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Electron ConfigurationsThe next irregularity is 5s and 4d are close in energy

1s<2s<2p<3s<3p<4s≈3d<4p<5s≈4d

Still more irregularities exist with higher energy orbitals.

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Rules for Writing e- Configs1. Determine the # of e-

the atom has (atomic #)

Fluorine as an example has 9 electrons

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Rules for Writing e- Configs

2. Fill orbitals in order of increasing energy (follow the diagram on the next slide)

S orbital – 2 e-P orbitals – 6 e-D orbitals – 10 e-F orbitals – 14 e-

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Rules for Writing e- Configs

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Rules for Writing e- Configs The configuration for F is

1s22s22p5 3. Make sure the total

number of e- in the config match the atomic number

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Electron ConfigurationsTells us how the 16 e- of S are configured

The electron configuration for S is

S=1s22s22p63s23p4

Each s orbital has 2 e-, each p orbital can have 6 e- (2 per orbital)

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Electron Configurations Do the electron configurations for the following elements…

O, Ar, Ca, V, Sr, and SnRemember the d orbital is off by 1 E level

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The f orbitalIf you end in the f orbital – make sure you indicate d1 first.

If you go through the f orbital, count the s orbital, then all of the f then go to d and beyond.

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Electron ConfigurationsThere are still some irregularities with higher energy orbitals.

Chromium is an example of thisCr =1s22s22p63s23p64s13d5

There is one unfilled d orbital and a filled s orbital

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Electron ConfigurationsThere are e- configs listed in the p.t. in the back of your book.

These are the ground state configs of the isolated atoms in the gas phase.

Under other conditions, the configs could be different

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Orbital DiagramsHund’s Rule says the maximum stability for e- is when you have the maximum number of unpaired electrons when they have the same quantum number.

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Orbital DiagramsUse boxes to show e- locationBoron = 1s22s22p1

The orbital diagram looks like…

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1s

2s

{2p

Orbital DiagramsWhat is the Carbon orbital diagram.

The config is 1s22s22p63s2

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Electron ConfigurationsTo save space, some configurations are written like

S = [Ne]3s23p4

This means take neon’s configuration and add 3s23p4 to the end of it.

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