Answer:
inert
Explanation:
Helium is an inert gas. All the substances on this earth react because they want to be stable and stability is achieved only when they acquire a stable electronic configuration. Examples of stable electronic configurations are 2/ 2,8/ 2,8,8 etc. It is evident from the example that a stable electronic configuration is achieved when the outermost shell i.e. valence shell of an atom is full i.e. if it has 2 or 8 electrons. If the outermost shell of an atom does not have either of these two electrons then it means that it is reactive and need to either gain electrons or loose electrons so that outermost shell may have stable electronic configuration. Whether an atom will loose or gain electrons to achieve stable electronic configuration it totally depends upon ease of either gaining or loosing.
For example, Sodium (Na) has in total 11 electrons so its electronic configuration will be 2,8,1. In the valence shell it has 1 electron but to have 8 electrons in the valence shell, it needs to either gain 7 electrons or loose 1 electron. But, since loosing 1 electron is easier, it will loose 1 rather than gaining 7.
But when it comes to chlorine (Cl) which has 17 electrons in total and therefore has an electronic configuration 2,8,7 it will gain 1 electron rather than loosing 7 because gaining 1 electron is much easier than loosing 7 electrons and ultimately it will have 8 electrons in outermost shell which is stable configuration.
A substance is considered as inert if it does not need to react with any other atom because it already has stable electronic configuration. Helium has 2 electrons in its outermost shell which is already a stable electronic configuration therefore it is considered as inert because it does not need to react with any other atom.
