(50 points)

an unknown metal displaces aluminum (al) from solution but does not displace sodium (na). using the activity series, determine the unknown metal.

a.) calcium
b.)zinc
c.)manganese
d. magnesium

The answer is D, Magnesium.

For an anion or cation(negatively or positively charged ion) to replace a similarly-charged ion, it needs to be more active.  This means that only a cation that is above Zinc on the reactivity scale can replace Zinc.

Since Aluminum was displaced in the reaction, that means the unknown metal is more reactive.  
This means that our options are:
Lithium, Potassium, Calcium, Sodium, and Magnesium.

But because the metal did not displace the Sodium, it must be less reactive than it.
This gives us a new list of metals that are less-reactive than Sodium:
Magnesium, Aluminum, Zinc, Iron, Nickel, etc.

The only metal on both lists is Magnesium, and that means that your answer is D, Magnesium.

High-energy and unstable.