Isomerism is the phenomenon whereby certain compounds, with the same molecular formula, exist in different forms owing to their different organisations of atoms. The concept of isomerism illustrates the fundamental importance of molecular structure and shape in organic chemistry.
By signing the nuclear deal, Iran has given up that nuclear umbrella for as long as two decades and made the case for the United States to further strengthen the conventional military capacities of Israel and the GCC.
Image caption Iran is one of Syria's closest allies. Iran, a crucial ally of President Assad, was unexpectedly invited to join the start of the Geneva II conference by UN Secretary General Ban Ki-moon on 19 January. Mr Ban said he strongly believed Iran had to be part of the solution to the crisis in Syria.