▁Auf bau ▁principle ▁causes ▁elements ▁of ▁the ▁period ▁to ▁put ▁electrons ▁onto ▁ 4 s , ▁ 3 d , ▁and ▁ 4 p ▁sub sh ells , ▁in ▁that ▁order . ▁However , ▁there ▁are ▁exceptions , ▁such ▁as ▁chrom ium . ▁The ▁first ▁eleven ▁elements — K , ▁Ca , ▁and ▁transition ▁met als — have ▁from ▁ 1 ▁to ▁ 1 1 ▁val ence ▁electrons ▁respectively , ▁which ▁are ▁placed ▁on ▁ 4 s ▁and ▁ 3 d . ▁ ▁Tw elve ▁electrons ▁over ▁the ▁electron ▁configuration ▁of ▁arg on ▁reach ▁the ▁configuration ▁of ▁z inc , ▁namely ▁ 3 d 1 0   4 s 2 . ▁From ▁this ▁element , ▁the ▁filled ▁ 3 d ▁sub shell ▁effectively ▁withdraw s ▁from ▁chem istry ▁and ▁the ▁subsequent ▁tr end ▁looks ▁much ▁like ▁tr ends ▁in ▁the ▁periods ▁ 2 ▁and ▁ 3 . ▁The ▁p - block ▁elements ▁of ▁period   4 ▁have ▁their ▁val ence ▁shell ▁composed ▁of ▁ 4 s ▁and ▁ 4 p ▁sub sh ells ▁of ▁the ▁fourth ▁() ▁shell ▁and ▁obey ▁the ▁oct et ▁rule . ▁ ▁For ▁quantum ▁chem istry ▁namely ▁this ▁period ▁sees ▁transition ▁from ▁the ▁simplified ▁electron ▁shell ▁parad ig m ▁to ▁research ▁of ▁many ▁differently - sh aped ▁sub sh ells , ▁relative ▁disposition ▁of ▁whose ▁energy ▁levels ▁is ▁govern ed ▁by ▁inter play ▁of ▁various ▁physical ▁effects . ▁The ▁period ' s ▁s - block ▁met als ▁put ▁their ▁differenti ating ▁electron ▁onto ▁ 4 s ▁having ▁vac an