Atoms are made up of a positively charged nucleus surrounded by a cloud of negatively charged electrons. Nuclei are very dense and extremely small, they contains more that 99.9% of the mass of an atom and are ten thousand times smaller than an atom!
The nucleus is a collection of particles called protons, which are positively charged, and neutrons, which are electrically neutral. Protons and neutrons are in turn made up of particles called quarks.
The chemical element of an atom is determined by the number of protons, or the atomic number, Z, of the nucleus. The element oxygen has an atomic number Z=8, while carbon has Z=6. The atomic mass of the nucleus is given by, A=Z+N, where, N, is the number of neutrons in the nucleus.
Different isotopes of an element have different numbers of neutrons in their nucleus. For example, the stable isotope carbon-12, the most common type of carbon in the human body, has Z=6 and N=6, while carbon-14, the radioactive isotope used in carbon dating, has Z=6 and N=8. There are less than 300 stable nuclei and over 3000 unstable radioactive nuclei.