Metals consist of giant structure of atoms arranged in a regular pattern. The electrons in the outer shell of metal atoms are shared , and are free to move throughout the structure. Therefore the structure is formed by positive charged metal ions held together by a 'sea' of delocalised.
Because I (iodide) is better leaving group than Cl, so it will leave when this molecule is reacted with strong base (sodium tert-butyl oxide) giving the elimination product provided in picture<span />
