Atomic History Dalton’s Atomic Theory – 1803 • All elements are composed of indivisible units called atoms. • All atoms of the same element are identical. • All atoms of different elements are different. • Compounds are formed by joining atoms of 2 or more elements in a definite whole number ratio • Changes made to Dalton’s theory in the modern theory – Atoms are divisible (made of smaller particles) – Isotopes of elements are the same chemically, but have different mass numbers • Isotopes have the same protons and electrons but different neutrons Discovery of Electrons • William Crooke – invented Crookes tube or Cathode Ray tube. – used electrically charged metal cathodes to send charged gas molecules across a vacuum tube. Gases would emit light at the opposites end of the tube. Believed the emission was cathode rays. Discovery of Electrons • JJ Thomson – used Crookes tube to show that cathode rays were actually negatively charged particles – used a wide variety of metals and gases and determined that all atoms contained these negatively charged particles – discovery of electrons led Thomson to form “plum- pudding model” • “Plum Pudding Model” – Equal, even distribution of protons and electrons • Rutherford’s Model (Lord Rutherford) – a small, dense, positively charged nucleus surrounded by seemingly empty space with negatively charged electrons orbiting the nucleus. – “Gold Foil Experiment” • Projected + Alpha particles at a thin gold foil surrounded by a ZnS screen that sparked when struck by Alpha particles • Most of the alpha particles passed straight through the foil, some were slightly deflected, while only a few were deflected by 90° or more • Results indicated that the positive charge was small and dense and the electrons were spread out in empty space. • *Rutherford could not explain how the electrons could orbit the nucleus without losing energy • Bohr Model (Niels Bohr 1913) – electrons are found in specific energy levels in which they can travel w/out radiating energy. – The lowest energy electrons are found closest to the nucleus. The further from the nucleus an electron is, the more energy it has. – When all electrons of an atom are in the lowest available energy levels the atom is in the ground state and is stable. • Charge – Cloud Model – Electrons are actually clouds of negative charge that are moving at speeds close to the speed of light. – The model shows the most probable location of an electron .