Question

why can a hydrogen atom form a covalent bond without fulfilling the octet rule? hydrogens valence shell can hold only up to two electrons. hydrogens valence shell never needs to be full to be stable. the octet rule only applies to bonds formed by trading electrons. the octet rule only applies to alkali metals and alkaline earth metals.

Explanation:

Hydrogen has only one electron shell (the first shell). The first electron shell can hold a maximum of 2 electrons. The octet rule (which states that atoms tend to gain, lose or share electrons to have 8 electrons in their valence shell) is a generalization. For hydrogen, since its valence shell (the only shell it has) can only hold 2 electrons, it follows the "duet rule" (aiming for 2 electrons in its valence shell). So, when hydrogen forms a covalent bond (shares an electron), it can achieve a stable electron configuration (2 electrons in its valence shell) without following the octet rule (which is about 8 - electron valence shell).

Answer:

Hydrogen’s valence shell can hold only up to two electrons.