The smallest unit of a substance that still acts like the main substance. Gamma rays from a nuclear explosion produce high energy electrons through compton scatteringfor high altitude nuclear explosions these electrons are captured in the earths magnetic field at altitudes between twenty and forty kilometers where they interact with the earths magnetic field to produce a coherent nuclear electromagnetic pulse nemp which lasts about one millisecond. Water is a substance and one molecule of water is made of two hydrogen atoms and one oxygen atom which we write as h2o.