Ionic compounds

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Super Conductivity

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                                  Super Conductivity   https://en.m.wikipedia.org/wiki/File:Periodic_table_with_superconducting_temperatures.jpg                                Metals are good conductors of electricity, and their conductivity increases as the temperature is lowered. In 1911 the Dutch scientist Heike Kamerlingh Oimes discovered that metals such as Hg and Pb became superconductors at temperatures near absoiute zero.  A superconductor has zero or almost zero electrical resistance: It can therefore carry an electric current without losing energy, and in principle the current can flow for ever. There is a critical temperature.  That which the resistance drops sharply and superconduction occurs. Later, Meissner and Ochsen feld found that some superconducting materials will not permit a magnetic ...
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Conductors. insulators and semiconductors

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  In electrical conductors (metals), either the valence band is oniy partly fu1l, or the valence and conductiori bands overlap. There is therefore no significant gap between filled and un_filled MOs, and perturbation can occur readily. In insulators (non-metals), the valence band is full, so perturbation within the band is impossible, and there is an appreciable difference in energy (called the band gap) between the valence band and the next empty band. Electrons cannot therefore be promoted to an empty level where they could move freely. Intrinsic semiconductors are basically insulators, where the energy gap between adjacent bands is sufficiently small for thermal energy to be able to promote a small number of electrons from the full valence band to the empty conduction band. Both the promoted electron in the conduction band and the unpaired electron left in the valence band can conduct electricity. The conductivity of semiconductors increases with temperature, because the number ...
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Size of atoms

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  The size of atoms decreases fr()m left to right across a period in the periodic table. For example. on moving from lithium to beryllium one extra positive charge is added to the nucleus, and an extra orbital electron is also added. Increasing the nuclear c:harge results in all of the orbital electrons being pulled closer to the nucleus. In a given period, the alkali metal is the largest atom and the halogen the smallest. When a horizontal period contains ten transition elements the contraction in size is larger, and when in addition there are 14 inner transition eleme11ts in a horizontal period, the contraction in size is even more marked. On descending a group in the periodic table such as that containing lithium. sodium, potassiUm, rubidium and caesium, the sizes of the atoms increase due to the effect of extra shells of electrons being added: this outweighs the effect of increased nuclear charge.
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