Why is Fluorine more reactive than Argon?
The short answer is that argon is already in a very stable electron configuration, so it takes a lot of energy to either add or remove an electron. Fluorine is in a very unstable configuration, so it is very easy to add an electron. More details below.
Argon has a full outermost electron shell of 8 electrons. Its electron configuration is 2, 8, 8.
This configuration is stable. We call it a 'full valence shell'. It takes a lot of energy to either add an extra electron to a new, higher energy level to give
Because it takes a large amount of energy to cause argon to react, it is very chemically stable - not very reactive.
Fluorine, on the other hand, does not have a full outer shell. Its configuration is 2, 7. We sometimes say that it 'wants' to have a full outer shell, but atoms don't really have minds and desires. It's more accurate to say that having a full outer shell is a more stable configuration. By adding one electron, it becomes
Fluorine reacts very easily to gain an electron to fill its outermost shell, and this is why it is much more reactive than argon.