Isotopes
Key Idea 1: Atomic Structure
What is the structure of an atom?
What Do We Know?
Atoms consist of a central nucleus, containing positively charged protons (m=1.67x10-27 kg) and neutral neutrons (m=1.67x10-27 kg), surrounded by negatively charged electrons (m=9.109x10-31> kg). Protons and neutrons have almost the same mass, which is much larger than the mass of an electron. Therefore, the vast majority of the mass of an atom is contained in the nucleus. However, the outer electrons occupy the majority of the physical space taken up by an atom.
Neutral atoms have the same number of protons as electrons. The numbers of protons and electrons govern the many properties of the atom.
Different elements are defined by the atomic number of protons contained in their atoms. The number of protons of a specific element is called the atomic number and is given the symbol, Z. The atomic number of each element is indicated on the periodic table.
Much of our knowledge of atomic structure was discovered through the famous Rutherford experiment. In this experiment, Ernest Rutherford fired a beam of positively-charged helium nuclei (called alpha particles) at a thin piece of gold foil. Most of these particles went through the foil, and some were deflected by over 90 degrees. This information led Rutherford to conclude that atoms must be mostly empty space, with a positively charged nucleus.