Question

Why He2 is not formed?

Answer 1

Bcoz helium is a noble gas and its Valence shell is filled with 2 electrons .

Answer 2

An atom reacts only when it needs to achieve it's lowest state of energy in stable octet configuration or duplet in case it has only one shell.
Here, helium has 2 electrons in it's only shell which is duplet structure so it is in lowest state of energy. Therefore it won't react and is categorised as a Noble gas. It will neither react with it's own atom nor with the atom of any other element.

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