Which metals react so vigorously with oxygen that they catch fire if kept in the open?

The correct answer is Potassium and sodium.

Key Points

• ​Potassium and sodium are highly reactive alkali metals that react so vigorously with oxygen and moisture in the air that they can catch fire if exposed to the open environment.
• Both potassium and sodium react exothermically with oxygen, forming their respective oxides, and can ignite spontaneously due to the heat generated in these reactions

Additional Information

• Transition metals like iron and copper react with oxygen but at a much slower rate, often forming a protective oxide layer.
• Magnesium also reacts with oxygen, especially when heated, producing a bright white flame and forming magnesium oxide.