▁of ▁mag nes ium . ▁This ▁small ▁radius ▁and ▁high ▁weight ▁cause ▁it ▁to ▁be ▁expected ▁to ▁have ▁an ▁extremely ▁high ▁density ▁of ▁around ▁ 4 6   g · cm − 3 , ▁over ▁twice ▁that ▁of ▁os m ium , ▁currently ▁the ▁most ▁dense ▁element ▁known , ▁at ▁ 2 2 . 6 1   g · cm − 3 ; ▁un hex quad ium ▁should ▁be ▁the ▁second ▁most ▁dense ▁element ▁in ▁the ▁first ▁ 9 ▁periods ▁of ▁the ▁periodic ▁table , ▁with ▁only ▁its ▁neighbour ▁un he xt rium ▁( element ▁ 1 6 3 ) ▁being ▁more ▁dense ▁( at ▁ 4 7   g · cm − 3 ). ▁Met all ic ▁un hex quad ium ▁should ▁be ▁quite ▁stable , ▁as ▁the ▁ 8 s ▁and ▁ 8 p 1 / 2 ▁electrons ▁are ▁very ▁deeply ▁buried ▁in ▁the ▁electron ▁core ▁and ▁only ▁the ▁ 7 d ▁electrons ▁are ▁available ▁for ▁bond ing . ▁Met all ic ▁un hex quad ium ▁should ▁have ▁a ▁very ▁large ▁coh es ive ▁energy ▁due ▁to ▁its ▁c oval ent ▁b onds , ▁most ▁probably ▁resulting ▁in ▁a ▁high ▁mel ting ▁point . ▁ ▁The oret ical ▁interest ▁in ▁the ▁chem istry ▁of ▁un hex quad ium ▁is ▁largely ▁motiv ated ▁by ▁theoretical ▁predictions ▁that ▁it , ▁especially ▁the ▁is ot ope ▁ 4 8 2 U h q ▁( with ▁ 1 6 4 ▁pro tons ▁and ▁ 3 1 8 ▁neut rons ), ▁would ▁be ▁at ▁the ▁center