Chemsheets AS 1009 (Electron arrangement)

Electrons are arranged in electrons shells (energy levels).
The shells have sub-shells (sub-levels).
Each shell/sub-shell is made up of electron orbitals which can each hold 2 electrons.
Shells, sub-shells & orbitals
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Each sub-level consists of electron orbitals (region of space in which the electron spends most of its time).
Each orbital can hold 2 electrons with opposite spins (one electron spins clockwise and one anticlockwise).
Orbitals are regions of space that electrons are most likely to be in.
Orbitals
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s orbital
p orbital
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The Orbitron
http://winter.group.shef.ac.uk/orbitron/AOs/1s/index.html
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Orbitals
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Electrons enter the lowest energy orbital available.
Aufbau Principle
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Electrons prefer to occupy orbitals on their own, and only pair up when no empty orbitals of the same energy are available .
Hund’s Rule
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e.g. silicon 14 e- 1s2 2s2 2p6 3s2 3p2

e.g. calcium 20 e- 1s2 2s2 2p6 3s2 3p6 4s2

The highest energy electrons are lost when an ion is formed.
Note that 4s electrons are lost before 3d (as once 4s and 3d are occupied, 4s moves above 3d).
Ions
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e.g. Ca2+ 18 e- 1s2 2s2 2p6 3s2 3p6
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Cu and Cr do not have the expected electron structure.
Cr = 1s2 2s2 2p6 3s2 3p6 4s1 3d5 NOT 1s2 2s2 2p6 3s2 3p6 4s2 3d4
Cu = 1s2 2s2 2p6 3s2 3p6 4s1 3d10 NOT 1s2 2s2 2p6 3s2 3p6 4s2 3d9
Cu & Cr
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Evidence for how the electrons are arranged in atoms comes from ionisation energies.
1st ionisation energy = energy required to remove one electron from each atom in a mole of gaseous atoms producing one mole of 1+ gaseous ions.
Note that 2nd ionisation energy is the energy required to remove the second electron (not both electrons).
e.g. 1st IE of Na: Na(g) → Na+(g) + e–
2nd IE of Na: Na+ (g) → Na2+(g) + e
Ionisation Energy
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Ionisation Energy
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1st ionisation energy (down group)
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1st ionisation energy (down group)
Atoms get bigger
More shielding
Therefore weaker attraction from nucleus to electron in outer shell
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1st ionisation energy (across period)
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1st ionisation energy (across period)
General trend
Increased nuclear charge (i.e. more protons)
Atoms get smaller
Therefore stronger attraction from nucleus to electron in outer shell
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1st ionisation energy (across period)
Group 2 → 3
Electron lost from Group 3 element is from p orbital, while that lost from Group 2 element is from s orbital.
p orbital is higher energy than s orbital, so easier to lose electron.
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1st ionisation energy (across period)
Group 5 → 6
Group 6 element loses electron from orbital with 2 electrons (p4)
Group 5 element loses electron from orbital with 1 electrons (p3)
Extra electron-electron repulsions make it easier to lose electron from p4 than p3.
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1st ionisation energy
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1st ionisation energy
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down a group
(group 0)

1st ionisation energy
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down a group
(group 1)

1st ionisation energy
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period 2
period 3
period 4
Across a period

1st ionisation energy
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End of period