nucleus
the very small center core of an atom
protons
have a positive charge and are in the nucleus
neutrons
have no charge and are found in the nucleus
electrons
move rapidly around the nucleus and have a negative charge
atomic number
the number of protons in its nucleus
isotopes
atoms with the same number of protons and a different number of neutrons
mass number
the sum of the protons and neutrons in the nucleus of an atom
model
may be a diagram, a mental picture, a mathematical statement, or an object that helps explain ideas about the natural world.