Ionisation Energies

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AS - Level Chemistry (7 - Periodicity) Flashcards on Ionisation Energies, created by Bee Brittain on 27/04/2016.
Bee Brittain
Flashcards by Bee Brittain, updated more than 1 year ago
Bee Brittain
Created by Bee Brittain over 8 years ago
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Question Answer
What is Ionisation energy? Ionisation energy measures how easily an atom loses electrons to form positive ions
Define the term, 'First Ionisation Energy' First Ionisation energy is the energy required to remove one electron from each atom in one mole of gaseous atoms of an element to form one mole of gaseous 1+ ions
Factors that affect ionisation energy - Atomic Radius - Nuclear Charge - Shielding
Atomic Radius The greater the distance between the nucleus and the outer electrons, the less the nuclear attraction. The force of attraction falls off sharply with increasing distance, so atomic radius has a large effect.
Nuclear Charge The more protons there are in the nucleus of an atom, the greater the attraction between the nucleus and the outer-shell electrons.
Shielding Electrons are negatively charged, so inner-shell electrons repel outer-shell electrons. This repulsion, called the shielding effect, reduces the attraction between the nucleus and the outer-shell electrons.
Two key patterns of first ionisation energy in the first 20 elements in the periodic table. 1) A sharp DECREASE in first ionisation energy between the end of one period and the start of another. He --> Li, Ne --> Na, Ar --> K 2) A general INCREASE in first ionisation energy across each period. H --> He, Li --> Ne, Na --> Ar
Trend in first ionisation energy down a group First ionisation energies decrease down a group. This is because although nuclear charge increases, Its effect it out-weighed by the increasing atomic radius as there are more shells and to a lesser extent, shielding also increases.
Trend in first ionisation energy across a period First ionisation energy shows a general increase across periods, however it does fall in two places. These falls also occur in the same positions on each period. This is due to sub-shells, their energies and how orbitals fill with electrons.
Trend in first ionisation energy across a period - Period 2 Two falls in period two. 1 between Be to B and the other between N to O. Apart from the two falls there is a general increase.
Trend in first ionisation energy across a period - Period 2, the fall from Be to B The fall from Be to B marks the start of filling the 2p sub-shell. The 2p sub-shell has a higher energy than the 2s sub-shell. Therefore in Boron, the 2p electron is easier to remove than one of the 2s electrons in Be. So, the first ionisation energy of B is < the first ionisation energy of Be.
Trend in first ionisation energy across a period - Period 2, the fall from N to O This fall marks the start of electron pairing in the p-orbitals of the 2p sub-shell. In N and O the highest the highest energy electrons are in a 2p sub-shell. In O, the paired electrons in one of the 2p orbitals repel one another, making it easier to remove an electron from nitrogen than oxygen. Therefore first ionisation energy of O < the first ionisation energy of N
Successive Ionisation Energies If there is a large increase suddenly in successive ionisation energies, it suggests that the latest electron was removed from a shell closer to the nucleus, with less shielding.
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