Why isn’t a ‘pure metal’ object automatically the best choice?
Your first guess might be that a material made from just one element should work best. In real life, you usually want the opposite. Many everyday metal objects are not made from a single pure metal because pure metals can react too easily with the air, bend too easily, or wear out too fast.
A reactive metal is one that tends to join in chemical changes readily, especially by reacting with substances such as oxygen. An unreactive metal is much less eager to do that, so it stays unchanged more easily.
Gold, platinum, and silver sit near the unreactive end of the activity series, so you can leave them exposed and they stay fairly unchanged. A lot of the metals you use every day are less stable in air at room temperature, so their surfaces change over time.
If you hear that a metal causes problems in pure form, that does not mean the pure metal is automatically poisonous. The bigger issue here is usually how easily it reacts, not toxicity.
You can see this with familiar objects. Copper on the Statue of Liberty did not stay shiny; over time it developed a dark green patina because copper reacted with water in the air through a pathway more complicated than simple oxidation. Iron in a steel adjustable wrench starts off bright and metallic-looking, but after long exposure to air it can turn reddish-brown as iron(III) oxide forms. Aluminium drink cans also begin shiny and silvery, then quickly gain an aluminium oxide coating.
The surprising bit is aluminium. Your aluminium object reacts with oxygen, but the reaction mostly stops near the surface. The first few atomic layers oxidize, and that thin oxide layer protects the metal underneath instead of letting the corrosion keep digging deeper.
A patina is a surface layer that appears after a metal has reacted with its surroundings over time, changing the metal’s colour and outer appearance.
Because oxygen in air can change metals, you often need protection such as coatings. But sometimes you also want to change the metal itself so it behaves better from the start. That is where alloying comes in.
Alloying is the process of mixing a metal with another metal or with carbon so the final material has properties better suited to a job.
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