Editing 2639: Periodic Table Changes
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|Move helium over here. It fits so nicely!||Helium is moved from the upper right corner to the second column next to hydrogen. However, the reason it is placed at the far-right Group 18 and not Group 2 is because it is a {{w|noble gas}}, rather than a reactive {{w|alkaline earth metal}}. You could say helium is in group 2 because it has two electrons in its outer shell, but normal periodic tables place it in group 18, the noble gases, with which it has far more in common. Hydrogen has similar problems being in group 1, as it's a non-metal and the elements below it are metals which don't have much in common with it chemically. There are periodic tables that show hydrogen floating above the periodic table for that reason. | |Move helium over here. It fits so nicely!||Helium is moved from the upper right corner to the second column next to hydrogen. However, the reason it is placed at the far-right Group 18 and not Group 2 is because it is a {{w|noble gas}}, rather than a reactive {{w|alkaline earth metal}}. You could say helium is in group 2 because it has two electrons in its outer shell, but normal periodic tables place it in group 18, the noble gases, with which it has far more in common. Hydrogen has similar problems being in group 1, as it's a non-metal and the elements below it are metals which don't have much in common with it chemically. There are periodic tables that show hydrogen floating above the periodic table for that reason. | ||
| − | Group 18 was not known at the time of the original table, which used I-VII (1-7) for the otherwise full-height columns, which turns out to reflect the number of free ('valence') electrons in their outer 'orbit', in one useful understanding of the atomic model. When added, the new final column was called either VIII (having a full outer shell) or Group 0 (having none free) and placed to the right of VII. It was originally thought that these so-called "noble gases" (no relation to Alfred Nobel of explosives and science prize fame | + | Group 18 was not known at the time of the original table, which used I-VII (1-7) for the otherwise full-height columns, which turns out to reflect the number of free ('valence') electrons in their outer 'orbit', in one useful understanding of the atomic model. When added, the new final column was called either VIII (having a full outer shell) or Group 0 (having none free) and placed to the right of VII. It was originally thought that these so-called "noble gases" (no relation to Alfred Nobel of explosives and science prize fame) were unable to react with other elements. However, xenon compounds were discovered in 1962, and many other {{w|noble gas compounds}} have been discovered since then under a variety of conditions. |
From the fourth row downwards the inserted block of Transition Metals represent a larger electron shell with more positions for free electrons and those elements to the right would have more electrons than those above them, by this measure, but their physical properties are still best dictated and aligned by the complement to the valence (i.e. the gap-size). The current numbering system shows the outer-shell electrons accurately in the rows where the ten new columns are, but the upper rows of columns 13-18 have ten more (or sixteen more, for Helium) than is the case. Similarly, once lanthanides and actinides are considered, the group number and outer-shell count becomes disconnected again in the opposite way. But it still seems useful enough to currently label in this manner under current {{w|IUPAC}} guidance. | From the fourth row downwards the inserted block of Transition Metals represent a larger electron shell with more positions for free electrons and those elements to the right would have more electrons than those above them, by this measure, but their physical properties are still best dictated and aligned by the complement to the valence (i.e. the gap-size). The current numbering system shows the outer-shell electrons accurately in the rows where the ten new columns are, but the upper rows of columns 13-18 have ten more (or sixteen more, for Helium) than is the case. Similarly, once lanthanides and actinides are considered, the group number and outer-shell count becomes disconnected again in the opposite way. But it still seems useful enough to currently label in this manner under current {{w|IUPAC}} guidance. | ||
