 As tetravalent transition metals , all three elements form various inorganic compounds , generally in the oxidation state of + 4 . For the first three metals , it has been shown that they are resistant to concentrated alkalis , but halogens react with them to form tetrahalides . At higher temperatures , all three metals react with oxygen , nitrogen , carbon , boron , sulfur , and silicon . Because of the lanthanide contraction of the elements in the fifth period , zirconium and hafnium have nearly identical ionic radii . The ionic radius of Zr4 + is 79 picometers and that of <unk> + is 78 pm . 
