5 ▁| - ▁style =" background :# c 0 ffff ;" ▁|| ▁ 3 6 ▁|| ▁Kr ▁|| ▁K rypt on ▁|| ▁No ble ▁gas ▁|| ▁[ Ar ] ▁ 3 d 1 0 ▁ 4 s 2 ▁ 4 p 6 ▁| } ▁ ▁(* ) ▁Exception ▁to ▁the ▁Mad el ung ▁rule ▁ ▁s - block ▁elements ▁ ▁Pot ass ium ▁ ▁Pot ass ium ▁( K ) ▁is ▁an ▁alk ali ▁metal , ▁placed ▁under ▁sod ium ▁and ▁over ▁rub id ium , ▁and ▁is ▁the ▁first ▁element ▁of ▁period ▁ 4 . ▁It ▁is ▁one ▁of ▁the ▁most ▁re active ▁elements ▁in ▁the ▁periodic ▁table , ▁therefore ▁usually ▁only ▁found ▁in ▁comp ounds . ▁It ▁tends ▁to ▁ox id ize ▁in ▁air ▁very ▁rapidly , ▁thus ▁account ing ▁for ▁its ▁rapid ▁reaction ▁with ▁o xygen ▁when ▁fresh ly ▁exposed ▁to ▁air . ▁When ▁fresh ly ▁exposed , ▁it ▁is ▁rather ▁sil very , ▁but ▁it ▁quickly ▁begins ▁to ▁t arn ish ▁as ▁it ▁react s ▁with ▁air . ▁It ▁is ▁soft ▁enough ▁to ▁be ▁cut ▁with ▁a ▁kn ife ▁and ▁it ▁is ▁the ▁second ▁least ▁dense ▁element . ▁Pot ass ium ▁has ▁a ▁relatively ▁low ▁mel ting ▁point ; ▁it ▁will ▁m elt ▁just ▁by ▁putting ▁it ▁under ▁a ▁small ▁open ▁fl ame . ▁It ▁also ▁is ▁less ▁dense ▁than ▁water , ▁and ▁can , ▁in ▁turn , ▁float . ▁ ▁Cal ci um ▁ ▁Cal ci um ▁( Ca ) ▁is ▁the ▁second ▁element ▁in ▁the ▁period .