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Chemical reactivity
The electrons, especially the outer electrons, determine chemical reactivity.
Chemical properties
The outer electrons and how they interact with other electrons (either gaining or losing them) are key in determining chemical reactivity.
Gold's chemical reactivity
Gold's outer electrons do not want to be lost or gain other electrons, making it unreactive.
Reactive metals with acids
Zinc and magnesium are very reactive because they readily give away their outer electrons.
Combustibility example
Methane (CH4) is highly reactive with oxygen and easily catches fire, making it highly combustible.
Water's reactivity with oxygen
Water does not catch on fire, so it is unreactive with oxygen.