06.Periodicity of Behavior;
       Sodium Through Argon
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For the second-row atoms they range from 1.0 for Li to 4.0 for F. In the third shell the span is only from 0.9 for Na to 3.0 for Cl.

Every thirdrow element is less electronegative than its counterpart above it, and so is more metallic. Na, for example, is a more reactive metal than Li. Li reacts fairly fast with water:

Li + H2O ------> Li+ + OH- + 1/2H2 (Some heat emitted)

The analogous reaction with sodium is so vigorous, enough heat

  Na + H2O ------> Na+ + OH- + 1/2H2 (Much heat emitted)

is emitted that the hydrogen gas is ignited in an explosion. Magnesium(Mg) metal is held together by two electrons per atom, as is beryllium metal, but magnesium loses these electrons more easily and therefore is more metallic.

In many ways magnesium resembles lithium more than it does beryllium.Among atoms with three outer-shell electrons, boron is nonmetallic, but aluniinum (AI), in the next row, is a metal.

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