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
adheres tightly to the surface of the metal and protects it from
(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
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.