FOREWORD THE COSMIC ELEMENTAL CONTEXT   The element hydrogen has the simplest atoms, each with just one  proton and one electron. The proton forms the nucleus, while the  electron orbits around it. All other elements have neutrons as well as  protons in their nucleus, such as helium, which is depicted in Figure  2.2. The positively charged protons tend to repel each other, and the  neutrons help to hold the nucleus together. The number of protons is the  atomic number , and the number of protons plus neutrons is the atomic mass .  For hydrogen, the atomic mass is 1 because there is one proton and no  neutrons. For helium, it is 4: two protons and two neutrons.  For most of the 16 lightest elements (up to oxygen) the number of  neutrons is equal to the number of protons. For most of the remaining  elements, there are more neutrons than protons, because extra neutrons  are needed to keep the nucleus together by overcoming the mutual  repulsion of the increasing numbers of prot...
 
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