Here's why:
* Valence electrons are the electrons in the outermost shell of an atom. These electrons are involved in chemical bonding.
* Hydrogen and helium are the only elements in the first period of the periodic table.
* Hydrogen has only one electron, which is its valence electron.
* Helium has two electrons, both of which are valence electrons.
* All other elements in the periodic table have more than two electrons, but some have inner shells filled with electrons.
* Atoms with a full outer shell are very stable, meaning they don't readily form bonds. For example, helium has a full outer shell with two electrons and is very unreactive.
Therefore, while some atoms have only one valence electron (like hydrogen), it is possible for an atom to have zero valence electrons when its outermost shell is empty. However, this is only the case for atoms with a completely filled inner shell.
