The reactiveness of an element depends on it’s electrons. I’m not sure if you have covered these yet, but these, along with other particles, are what make up an atom. They look a little like moons orbiting a planet, but are a bit more complicated than that!
Electrons can swap between different atoms, and depending on how easy this is to do, and what the other atoms are, create the reactivity of an element
If you are really keen on this I would advise you read up on it – this is just a very brief and simple overview, as it can get very complicated!