hydrogen and chlorine form a covalent bond. hydrogen has 1 valence electron and chlorine has 7 valence electrons. I was taught that 18 electrons are needed to fill third level of an atom. 2 electrons in the First level, then 8 electrons in the Second level, then 18 in the Third level. Chlorine has 7 valence electrons and needs 11 more electrons to be fill the third level. But the octet rule or the lewis dot structure doesn't help to know the bond between chlorine and hydrogen. Because 18 electrons representing the dots can't fit around the chemical symbol in the electron dot structure. I'm left confused understanding why chlorine is satisfied with one more electron from hydrogen but chlorine needs 11 more electrons to fill the third level not 8 in total but 18 in total. Please help explain my mistake.