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Ionisation Energy across a Period

Ionisation energy depends on:
  • the nuclear charge which depends on the number of protons
  • the shielding, energy of the quantum shell and distance from nucleus
  • the energy of the orbital
As we go across a period, the nuclear charge increases as one more proton is added. Although one more electron is also added, the electrons are in the same quantum shell.The increased nuclear charge is more significant than the increased shielding.Therefore the atomic radius decreases and the nuclear attraction for the outer shell electrons is greater. As a result the first ionisation energy increases across the period.

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Specification: Edexcel Chemistry (8CH0), AS-level, Topic 1: Atomic Structure and the Periodic Table. Topic: Atomic Structure.

Key terms in this lesson

shielding
The repulsion of outer shell electrons by electrons in filled inner shells, which reduces the nuclear attraction they feel.
ionisation energy
The energy needed to remove one electron from each atom in one mole of gaseous atoms, forming one mole of gaseous 1+ ions.

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