Ionization Energy - across a Period and down a Group TRENDS FOR IE 1 Group 0 elements (noble gases) have the highest ionisation energies: helium has the highest and the value decreases as it goes down the group. Noble Gases are very stable, thus electrons are held very tightly and are harder to remove. Group 1 elements (excluding hydrogen) have the lowest ionisation energies for the first electron removed. Be is higher than Li in Ionisation 3 than in ionisation 2 The next element in line will be have the highest ionisation energy as it goes down. For example, in ionisation 1 the highest will be He, then in ionisation 2 will be Li, then Be and so on. REASONS Across the period (horizontal) -> Increasing: The general trend is for ionisation energies to increase across a period. In period 2, all outer electrons are in the same level of orbital as the elements all have 2 electron shells. Therefore, electrons are all of the same sort of distances from the nucleus. However, the major difference is the increasing number of protons in the nucleus as you go across the period, from lithium to neon. This causes greater attraction between the nucleus and the electrons and so the ionisation energies increase. As the nuclear charge increases (nuclear charge numerically equal to atomic number), the attraction between the nucleus and the electrons increases and it requires more energy to remove the outermost electron and that means there is a higher ionisation energy. Down the groups (vertical) -> Decreasing: Elements down the groups have increasing number of valence shells. Hence, distances of electrons from the nucleus increase and this account for less pull exerted on the electrons. As such, IE decreases down the groups. As subsequent electrons are removed: Trend: Group 2 elements have the lowest IE for the second electron removed As subsequent electrons are removed the IE increases for each element. When removing an electron from a new shell (e.g. 2nd electron of element with 1 valence electron), the IE shows a significant increase.
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