1. Hydrogen has atomic number 1 and electronic configuration 1, that means it has 1 electron in its K shell and its duplet is short of one electron therefore, the valency of hydrogen is one.

2. On the other hand, chlorine has electronic configuration 2, 8, 7. Chlorine atom has 7 electrons in its outermost shell and requires one electron to complete its octet.
3. The two atoms, hydrogen and chlorine share one electron with each other. As a result, the electron duplet of hydrogen and octet of chlorine is complete and a covalent band is formed between them.