06.Periodicity of Behavior;
       Sodium Through Argon
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       Relative Reactivity of Metals

Metals generally become more reactive as the number of electron shells increases, because their outer electrons are held more weakly and thus are lost more easily.

The increase in reactivity with water, from Li to Na, already has been mentioned. Metals also become less reactive as atomic number increases within a shell.

Sodium burns in air with an intense yellow color, which is also seen in sodium vapor lamps. Magnesium burns readily enough that it is used in photoflash bulbs and flares, and must be handled with care when used as a structural metal in aircraft.

In contrast, aluminum is relatively inert, in part because it is intrinsically less reactive than sodium, but mainly because its oxide, Al2O3, adheres tightly to the surface of the metal and protects it from further corrosion.

(One of the curses of iron is that its oxide, Fe2O3, does not stick to the surface of the metal. It flakes away as rust, continually exposing fresh metal to attack by oxygen.)


In some ways, magnesium is the ideal metal for aircraft. It is less reactive than Li and Na, reasonably strong (thanks to its two electrons per atom), more ductile and easier to machine than the ultra-light but partly covalent Be, and lighter than nearly every other metal.

Aluminum runs a close second to magnesium, with the great advantage of being less combustible or reactive with oxygen because of its protective coating Of Al2O3.

Neither magnesium nor aluminum is active enough to react with water and to decompose it at room temperature, although any attempt to put out a magnesium flare or incendiary bomb with water will lead to the reaction of magnesium with steam to generate hydrogen, followed by an explosion.

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