9701 · 1.4
Ionisation energy flashcards
Revision flashcards for Cambridge 9701 Ionisation energy (syllabus 1.4). Flip, recall, then mark a real past-paper question.
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Define First Ionisation Energy.
The energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions. Units: kJ mol⁻¹.
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What are the three main factors affecting ionisation energy?
1. Nuclear charge (number of protons). 2. Atomic radius (distance of outer electron from nucleus). 3. Electron shielding (repulsion from inner shell electrons).
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Why does first IE generally increase across a period?
Nuclear charge increases, while atomic radius decreases and shielding remains relatively constant. This leads to a stronger electrostatic attraction between the nucleus and the outer electron, requiring more energy to remove it.
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Why does first IE decrease down a group?
The number of electron shells increases, leading to a larger atomic radius and increased shielding. These factors outweigh the increased nuclear charge, weakening the attraction for the outer electron.
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Write the general equation for the first ionisation energy of an element X.
X(g) → X⁺(g) + e⁻
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Explain the drop in first IE from Group 2 to Group 3.
The electron removed from a Group 3 element is in a p-subshell, which is at a higher energy level and further from the nucleus than the s-subshell of the Group 2 element. This makes it easier to remove.
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Explain the drop in first IE from Group 5 to Group 6.
In a Group 6 element, the electron is removed from a p-orbital containing a pair of electrons. The repulsion between these paired electrons (spin-pair repulsion) makes it easier to remove one of them compared to the singly-occupied orbitals in a Group 5 element.
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What are successive ionisation energies?
The energies required to remove electrons one after another from an atom or ion. For example, the second IE is the energy to remove an electron from a 1+ ion.
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Why are successive ionisation energies always larger than the previous one?
Each subsequent electron is being removed from an increasingly positive ion. The greater net positive charge results in a stronger electrostatic attraction to the remaining electrons, requiring more energy for removal.
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How can successive IE data identify an element's group?
A large jump in successive ionisation energies indicates the removal of an electron from a new, inner principal quantum shell. The number of electrons removed before this jump corresponds to the number of valence electrons, and thus the group number.
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What state symbols are essential in ionisation energy equations?
The gaseous state symbol (g) is required for both the atom and the resulting ion, as ionisation energies are measured for isolated particles in the gas phase.