system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: What determines if a metal produces an oxide layer?
gpt
Firstly how reactive it is to oxygen. A metal like gold is practically inert and so won’t readily oxidise in air. Where as something like sodium or aluminium will oxidise very quickly.

Secondly the physical properties of the oxide. Aluminium oxide isn’t water soluble and is fairly hard so sticks to the metal very well, on the other hand rust on iron is very weak and will be quickly eroded, revealing more iron to oxidise and losing much of the material.