Читать книгу Astrobiology - Charles S. Cockell - Страница 36

In the nucleus of any given atom, the total number of protons is given by Z. This is also called the atomic number. The number of protons defines the element. So, for instance, in the biologically important atom carbon, the number of protons is 6. The element has an atomic number of 6. This defines it as the element carbon.

The nucleus also contains neutrons (a total number N). For any given atom, the atomic mass is the total number of protons and neutrons. In all stable elements, there are at least an equal number of neutrons as protons, and in many elements there are more neutrons. An exception is hydrogen, which contains just one proton. Returning to carbon, most carbon atoms have 6 neutrons, so with its 6 protons this gives it an atomic mass number of 12. Protons, because they are positively charged, tend to repel each other. You might think that this would cause the nucleus to fall apart. However, the nucleus is held together by the strong nuclear force that operates over very small distances and overwhelms the electrostatic repulsion, explaining why the ball of positively charged protons in the atomic nucleus remains together.