▁ch lor ine ▁and ▁i od ine , ▁such ▁as ▁first ▁ion isation ▁energy , ▁electron ▁aff inity , ▁ent hal py ▁of ▁dis soci ation ▁of ▁the ▁X 2 ▁mole c ule ▁( X ▁= ▁Cl , ▁Br , ▁I ), ▁ion ic ▁radius , ▁and ▁X – X ▁bond ▁length . ▁The ▁vol at ility ▁of ▁b rom ine ▁acc ent u ates ▁its ▁very ▁pen etr ating , ▁ch oking , ▁and ▁un ple asant ▁od our . ▁ ▁All ▁four ▁stable ▁hal og ens ▁experience ▁inter m ole cular ▁van ▁der ▁Wa als ▁forces ▁of ▁att raction , ▁and ▁their ▁strength ▁increases ▁together ▁with ▁number ▁of ▁electrons ▁among ▁all ▁hom on uc lear ▁di atomic ▁hal ogen ▁mole cules . ▁Thus , ▁the ▁mel ting ▁and ▁bo iling ▁points ▁of ▁b rom ine ▁are ▁intermediate ▁between ▁those ▁of ▁ch lor ine ▁and ▁i od ine . ▁As ▁a ▁result ▁of ▁the ▁increasing ▁mole cular ▁weight ▁of ▁the ▁hal og ens ▁down ▁the ▁group , ▁the ▁density ▁and ▁he ats ▁of ▁fusion ▁and ▁v apor isation ▁of ▁b rom ine ▁are ▁again ▁intermediate ▁between ▁those ▁of ▁ch lor ine ▁and ▁i od ine , ▁although ▁all ▁their ▁he ats ▁of ▁v apor isation ▁are ▁fairly ▁low ▁( leading ▁to ▁high ▁vol at ility ) ▁thanks ▁to ▁their ▁di atomic ▁mole cular ▁structure . ▁The ▁hal og ens ▁dark en ▁in ▁colour ▁as ▁the ▁group ▁is ▁desc ended : ▁flu or ine ▁is ▁a ▁very ▁pale ▁yellow ▁gas , ▁ch lor