Atoms: Development of the Atomic Theory Democritus 460 BC - Greek philosopher proposes the existence of the atom He pounded materials until he made them into smaller and smaller parts He called them atoma which is Greek for “indivisible”. Democritus His Theory: All atoms: Are small hard particles Are made of a single material formed into different shapes and sizes Are always moving, and they form different materials by joining together John Dalton 1803 - British chemist; elements combine in specific proportions to form compounds Solid Sphere Model or Bowling Ball Model Proposed by John Dalton John Dalton His Theory: All substances are made of atoms that cannot be created, divided, or destroyed. Atoms join with other atoms to make new substances. Atoms of the same element are exactly alike, and atoms of different elements are different in mass and size. J.J. Thomson 1897 - English chemist and physicist; discovered 1st subatomic particles Plum Pudding Model or Raisin Bun Model Proposed by J.J. Thomson J.J. Thomson His Theory: Atoms contain negatively charged particles called electrons and positively charged matter. Created a model to describe the atom as a sphere filled with positive matter with negative particles mixed in Referred to it as the plum pudding model Ernest Rutherford 1912 - New Zealand physicist discovered the nucleus Nuclear Model Proposed by Ernest Rutherford Ernest Rutherford His Theory: Small, dense, positively charged particle present in nucleus called a proton Electrons travel around the nucleus, but their exact places cannot be described. Niels Bohr 1913 - Danish physicist; discovered energy levels Bohr Model or Planetary Model Proposed by Niels Bohr Niels Bohr His Theory: Electrons travel around the nucleus in definite paths and fixed distances. Electrons can jump from one level to a path in another level. Erwin Shrodinger 1924 - Austrian physicist; developed the electron cloud model Electron Cloud Model Proposed by Erwin Schrodinger Erwin Shrodinger His Theory: The exact path of electrons cannot be predicted. The region referred to as the electron cloud, is an area where electrons can likely be found. James Chadwick 1932 - English physicist; discovered neutrons His Theory: Neutrons have no electrical charge. Neutrons have a mass nearly equal to the mass of a proton. Unit of measurement for subatomic particles is the atomic mass unit (amu). Modern Theory of the Atom Atoms are composed of three main subatomic particles: the electron, proton, and neutron. Most of the mass of the atom is concentrated in the nucleus of the atom. Modern Theory of the Atom The protons and neutrons are located within the nucleus, while the electrons exist outside of the nucleus. In stable atoms, the number of protons is equal to the number of electrons. Modern Theory of the Atom The type of atom is determined by the number of protons it has. The number of protons in an atom is equal to the atomic number. Modern Theory of the Atom The sum of the number of protons and neutrons in a particular atom is called the atomic mass. Valence electrons are the outermost electrons.