oxygen. Adjusting these figures, in nitrous oxide there is 80 g of oxygen for every 140 g of nitrogen, in nitric oxide there is about 160 g of oxygen for every 140 g of nitrogen, and in nitrogen dioxide there is 320 g of oxygen for every 140 g of nitrogen. 80, 160, and 320 form a ratio of 1:2:4. The respective formulas for these oxides are N2O, NO, and NO2. Isomerism Scientists discovered some substances have the exact same chemical content but different properties. For instance, in 1827, Friedrich Wöhler discovered that silver fulminate and silver cyanate are both 107 parts silver, 12 parts carbon, 14 parts nitrogen, and 12 parts oxygen (we now know their formulas as both AgCNO). In 1830 Jöns Jacob Berzelius introduced the term isomerism to describe the phenomenon. In 1860, Louis Pasteur hypothesized that the molecules of isomers might have the same composition but different arrangements of their atoms. In 1874, Jacobus Henricus van 't Hoff hypothesized that carbon atoms bond to up to four other atoms in a tetrahedral configuration. Working from that, he created hypothetical models for the molecular structures of various organic compounds, and it so happened that these models were able to predict how many isomers a substance could have. For instance, pentane (C5H12) has three possible configurations in van 't Hoff's way of modelling molecules, and there really are just three isomers of pentane in nature. Discovery of the electron In 1897, J. J. Thomson discovered that cathode rays are not electromagnetic waves but made of particles because they can be deflected by electrical and magnetic fields. He measured these particles to be 1,800 times lighter than hydrogen (the lightest atom). Thomson concluded that these particles came from the atoms within the cathode — they were subatomic particles. He called these new particles corpuscles but they were later renamed electrons. Thomson also showed that electrons were identical to particles given off by photoelectric and radioactive materials. It was quickly recognized that electrons are the particles that carry electric currents in metal wires. Thomson concluded that these electrons emerged from the very atoms of the cathode in his instruments, which meant that atoms are not indivisible as Dalton thought. Discovery of the nucleus J. J. Thomson thought that the negatively-charged electrons were distributed throughout the atom in a sea of positive charge that was distributed across the whole volume of