<text><span class="style10">lements and the Periodic Table (4 of 8)</span><span class="style7"></span><span class="style10">Group position and chemical reactivity</span><span class="style7">Hydrogen has one electron in the first principal energy level, while helium has two - the maximum capacity for this level. The possession of one extra electron may seem a trivial difference, but a world of difference separates hydrogen and helium; hydrogen is very reactive and forms compounds with many other elements; helium combines with nothing. These two elements are rather exceptional in all their chemical behavior and are given a small section of their own in the Table, above groups 17 and 18 of the </span><span class="style10">p</span><span class="style7">-block.Hydrogen and helium are placed on the far right of the Table so that the latter falls in the same group (group 18) as other elements - the so-called </span><span class="style26">noble gases</span><span class="style7"> - that have full outer shells. Thus below helium we find neon, another chemically unreactive gas, which has the second principal energy level filled and is said to have an </span><span class="style26">electron configuration</span><span class="style7"> of 2.8. Just as we find hydrogen, a highly reactive element, immediately to the left of helium, so we find another reactive element - fluorine (configuration 2.7) - to the left of neon. Fluorine (like the other elements in group 17 - the </span><span class="style26">halogens</span><span class="style7">) is one electron short of a full outer shell. Fluorine's tendency to combine with other elements in order to achieve a full (and so stable) outer shell makes it one of the most reactive of all the elements - so reactive that it will even combine with the noble gases krypton and xenon.</span></text>
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<text>ΓÇó QUANTUM THEORY AND RELATIVITYΓÇó ATOMS AND SUBATOMIC PARTICLESΓÇó CHEMICAL BONDSΓÇó CHEMICAL REACTIONSΓÇó METALS</text>
