▁elements . ▁The ▁periodic ▁table ▁is ▁laid ▁out ▁in ▁rows ▁to ▁illustrate ▁rec urr ing ▁( period ic ) ▁tr ends ▁in ▁the ▁chemical ▁behaviour ▁of ▁the ▁elements ▁as ▁their ▁atomic ▁number ▁increases : ▁a ▁new ▁row ▁is ▁begun ▁when ▁chemical ▁behaviour ▁begins ▁to ▁repeat , ▁meaning ▁that ▁elements ▁with ▁similar ▁behaviour ▁fall ▁into ▁the ▁same ▁vertical ▁columns . ▁The ▁fourth ▁period ▁contains ▁ 1 8 ▁elements ▁beginning ▁with ▁pot ass ium ▁and ▁ending ▁with ▁k rypt on   – ▁one ▁element ▁for ▁each ▁of ▁the ▁eigh teen ▁groups . ▁It ▁sees ▁the ▁first ▁appearance ▁of ▁d - block ▁( which ▁includes ▁transition ▁met als ) ▁in ▁the ▁table . ▁ ▁Properties ▁ ▁Every ▁single ▁one ▁of ▁these ▁elements ▁is ▁stable , ▁and ▁many ▁are ▁extremely ▁common ▁in ▁the ▁Earth ' s ▁cr ust ▁and / or ▁core ; ▁it ▁is ▁the ▁last ▁period ▁with ▁no ▁un stable ▁elements ▁at ▁all . ▁Many ▁of ▁the ▁transition ▁met als ▁in ▁period ▁ 4 ▁are ▁very ▁strong , ▁and ▁therefore ▁commonly ▁used ▁in ▁industry , ▁especially ▁iron . ▁Three ▁adjacent ▁elements ▁are ▁known ▁to ▁be ▁to xic , ▁with ▁ar sen ic ▁one ▁of ▁the ▁most ▁well - known ▁po isons , ▁selenium ▁being ▁to xic ▁to ▁humans ▁in ▁large ▁quantities , ▁and ▁b rom ine , ▁a ▁to xic ▁liquid . ▁Many ▁elements ▁are ▁essential ▁to ▁humans ' ▁surv ival , ▁such ▁as ▁calci um ▁being ▁what ▁forms ▁b ones . ▁ ▁At omic ▁structure ▁Progress ing ▁towards ▁increase ▁of ▁atomic ▁number , ▁the