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What are the two exceptions of block division in the periodic table? |
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Answer» Solution :(i) Helium has two electrons. Its electronic configuration is `1s^(2)`?. As per the configuration, is supposed to be placed in .s. block, but actually placed in `18^(th)` group which belongs to ‘p’ block. Because it has a completely filled valence shell as the other elements presenti `18^(th)` group. It also resembles with `18^(th)` group elements in other properties. Hence helium placed with other noble GASES. (ii) The other exception is hydrogen. It has only one s-electron and hence can be placed in group 1. It can also gain an electron to achieve a noble gas arrangement and hence it can behave as halogens (`17^(th)` group elements). Because of these ASSUMPTIONS, position of hydrogen BECOMES a SPECIAL case. Finally, it is placed separately at the top of the PERIODIC table. |
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