4、地球物质的化学组成 Rock and Minerals inearth Elements and Isotopes 4 With the exception of the simplest hydrogen atoms, all nuclei contain neutrons and the number of neutrons is similar to or somewhat greater than the number of protons. The number of neutrons in atoms of one element is not always the same. The sum of the number of protons and the number of neutrons in a nucleus is the atom's atomic mass number. Atoms of a given element with different atomic mass numbers-in other words, with the same number of protons but different numbers of neutronsare distinct isotopes of that element. Some elements have only a single isotope, while others may have ten or more. (The 环质 Environmental Geology环境地质学 Environmental Geology 4、地球物质的化学组成 Rock and Minerals in earth Elements and Isotopes 4. With the exception of the simplest hydrogen atoms, all nuclei contain neutrons, and the number of neutrons is similar to or somewhat greater than the number of protons. The number of neutrons in atoms of one element is not always the same. The sum of the number of protons and the number of neutrons in a nucleus is the atom's atomic mass number. Atoms of a given element with different atomic mass numbers—in other words, with the same number of protons but different numbers of neutrons—are distinct isotopes of that element. Some elements have only a single isotope, while others may have ten or more. (The