n Nucleus consists of protons and neutrons   n Protons are positively charged; neutrons are neutral - no charge   n They are about the same mass - on the order of 10-27 kg   n Neutrons and protons are collectively called nucleons; the different nuclei are referred to as nuclides.   n Number of protons: atomic number, Z   n Number of nucleons: atomic mass number, A   n Neutron number: N = A - Z    n A and Z are sufficient to specify a nuclide. This is accomplished using Z as a subscript to X and A as a superscript to X

n X is the chemical symbol for the element; it contains the same information as Z but in a more easily recognizable form.

n Nuclei with the same Z – so they are the same element – but different N are called isotopes.

n For many elements, several different isotopes exist in nature.

n Natural abundance is the percentage of a particular element that consists of a particular isotope in nature.

n Because of wave-particle duality, the size of the nucleus is somewhat fuzzy. Measurements of high-energy electron scattering yield: r  @  (1.2 x 10^-15 m)A^1/3

n Masses of atoms are measured with reference to the carbon-12 atom, which is assigned a mass of exactly 12u. A u is a unified atomic mass unit.

       1 u = 1.6605 x 10^-27 kg = 931.5 MeV/u