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Ionisation energies provide the best evidence for the existence of electron shells
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First Ionisation Energy (∆Hᵢₑ): first ionisation energy of an element is the energy needed to remove
one electron from one mole of gaseous atoms to form one mole of gaseous unipositive ions
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Three factors influence ionisation energies:
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Size of positive nuclear charge: force of attraction between negatively charged electrons and
positively charged protons in nucleus
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Distance of electron from nucleus: as principal energy levels of electron shells increases,
distance between nucleus and outermost electrons increases so forces of attraction between
the nucleus and outermost electrons decreases
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Shielding effect by electrons in filled inner shells: filled inner shells will shield positive
nuclear charge
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All elements in same period experience the same shielding effect
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This is because:
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Size of positive nuclear charge increases across period
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No additional shielding
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First ionisation energy equations of lithium and magnesium:
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Li (g) --> Li⁺ (g) + e⁻
Mg (g) --> Mg ⁺ (g) + e⁻

Magnesium has a higher ionisation energy than sodium because:
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Electron being removed experiences a greater positive nuclear charge
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Greater attraction of ion to lost electron
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3p orbital higher in energy than 3s
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First ionisation energy of sulphur is less than phosphorus despite sulphur having a larger nuclear
charge because:
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Pair of electrons in 3p orbital
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Trends down a group - ionisation energies decrease down a group
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Further from nucleus
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Weaker attraction of the ion to the lost electron
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As a result, successive ionisation energies show increase in value
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e
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Ca⁺ (g) --> Ca²⁺ (g) + e⁻
Second ionisation energy value always greater than first ionisation energy

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