The structure of the atom[IMAGE]Basic atomic particles: Atoms are made up of the following particles:Protons: Protons are positively charged particles with a mass equal to one unit of atomic mass. They are located in the nucleus at the center of the atom's structure. Neutrons: Neutrons are uncharged and have the mass of one atomic mass unit. They are found with protons at the center of the atom.Electrons: Electrons are negatively charged particles with a mass of 1/1846 of an atomic mass unit. They are arranged in shells around the central core. Useful definitions for atomic structure: Atomic number (Z): the number of protons in the nucleus of an atom, but it is also equal to the number of electrons in the same atom (because atoms do not carry an overall charge: the positives balance the negatives! ). Mass number, (A) (or atomic mass): The number of neutrons and protons in the nucleus of an atom of the element. The atomic number and mass number of a particular element can be found from the periodicity table. The atomic number is always displayed at the bottom left of the element symbol. The mass number is always displayed at the top left of the element symbol. This symbol shows that sodium atoms have 11 electrons orbiting a nucleus that contains 11 protons and (23 - 11=) 12 neutrons.[IMAGE]EGSodium has an atomic number of 11 (and therefore each sodium atom contains 11 electrons and 11 protons). The structure above represents the GCSE model of the atomic structure of sodium. Electrons orbit a central nucleus in well-defined electron shells.[IMAGE]Simple diagram of an atom: the sodium atom. The first electron shell (closest to the nucleus) can contain a maximum of 2 electrons. The second electron shell can contain a maximum of 8 electrons. The third electron shell can contain a maximum of 18 electrons.