Final answer:
Aluminum is not commonly used in metal hydroxide relaxers because of its tendency to form a protective oxide coating, making it less reactive. Whereas sodium and potassium Group 1 alkali metals and calcium, an alkaline earth metal, form strong bases and more soluble hydroxides suitable for use in relaxers.
Step-by-step explanation:
In the context of metal hydroxide relaxers, the type of metal that is not commonly used to form ionic compounds is aluminum. Sodium, potassium, and calcium are more reactive and suitable for forming ionic compounds that can be used in relaxers. Aluminum, on the other hand, tends to react with air to form a passivation protective oxide coating, which makes its compounds less reactive and hence less available for the specific application in hydroxide relaxers.
While Group 1 metals such as sodium and potassium react with water to form strong bases used in relaxers, aluminum does not fall under such reactive alkali metals. Calcium relaxers are also common because like sodium and potassium, calcium, which is an alkaline earth metal, can react with water to produce hydroxide ions. However, many of the salts formed by alkaline earth metals are insoluble in water, distinguishing them from the alkali metal hydroxides.