2. Scientists of Atomic Theory Unit 2

3. Democritus 460 B.C.- 370 B.C. This Greek philosopher was among the first to suggest the existence of atoms. Democritus to propose that matter consisted of tiny, indivisible, unchangeable particles called atoms.

4. John Dalton 1766 - 1844 English chemist, called the Father of the Atomic Theory because he proposed a 5-part atomic theory, still recognized today as amazingly accurate.

5. Dalton’s Atomic Theory 1. All matter is made of indivisible, indestructible atoms. 2.Atoms of a given element are identical in their physical and chemical properties. 3.Atoms of different elements have different physical and chemical properties (which obviously follows from #2).

6. Dalton’s Atomic Theory (Cont.) 4.Atoms of different elements combine in simple, whole-number ratios to form chemical compounds. 5.Atoms cannot be subdivided, created or destroyed when they are combined, separated or rearranged in chemical reactions.

7. Was Dalton entirely correct? No. There were problems with 3 of his theories. Atoms are made of smaller particles (#1). Today, atoms are split in nuclear reactions. (#1, #5). Not all atoms of an element are not identical (#2). (Isotopes & Allotropes)

8. J.J. Thomson 1856 - 1940 English chemist who worked with electricity in a cathode ray tube. Determined that cathode rays came from atoms and are negatively charged. Called them electrons

9. Plum Pudding Model Working with neon, he also described isotopes of elements. He also determined atoms also contained positively charged particles, protons. This lead him to proposed his Plum Pudding Model of the atom. A positively charged sphere with negatively charged electrons embedded within.

10. Ernest Rutherford 1871 - 1937 He performed the “Gold Foil Experiment” where a positively charged beam was shot through a thin sheet of gold foil. He found that most of the particles went straight through but some were deflected and a few actually bounced back toward the source!

11. Gold Foil Experiment

12. Rutherford determined that atoms have a small, dense, positively charged center which is called the nucleus. He had just found the location of protons, and proposed that name. He also proposed that an atom is mostly empty space. In fact, the nucleus occupies only 1/8000th of the space of an atom.

13. Rutherford’s Model His atomic model was of electrons around the edges of the atom and the nucleus in the center. Getting closer…

14. Max Planck 1858 - 1947 Developed the early “Quantum Theory” (later expanded by Albert Einstein) which said that energy comes in bundles or packets called Quanta. “Quanta” of radiant energy are often called Photons. His theory was used by the next scientist to describe the placement of electrons in an atom.

15. Nils Bohr 1885-1962 Rutherford’s student, Nils Bohr, used the quantum theory (from Planck) to modify the model into a Planetary Model where electrons are in fixed orbits around the nucleus. Electrons are in fixed orbits with 2 electrons in the first orbit and up to 8 in the second. Oxygen Atom

16. Robert Millikan Millikan is famous for the Oil Drop Experiment (1909), where he determined the mass of an electron to be ~1/2000 of the mass of a proton, and the exact charge of the electron.

17. Marie Curie 1867-1934 Marie & Pierre Curie coined the term “radioactivity”. Using pitchblende, they isolated 2 new elements, both radioactive. They named the elements Radium and Polonium, after her native Poland.

18. Marie & Pierre Curie (Cont). Marie found the powerful energy that showed up in radioactivity was a fundamental property of every atom of matter. Marie was the first women to receive a Nobel prize and she won twice! Marie died of aplastic anemia (caused by exposure to radiation), in 1934.Pierre died in an accident.

19. Walter Bothe 1891-1957 He used Irene Curie’s research where she bombarded beryllium with alpha particles. Bothe found a high- penetrating beam was formed but could not identify the beam. He found the first evidence of the neutron.

20. James Chadwick 1891 - 1974 Chadwick is credited with discovering the neutron. He did similar experiments and found these particles to have the same mass as a proton but was not electrically charged. Since they were neutral, he named these particles neutrons.

21. Dimitri Mendeleev 1834 - 1907 In 1863 he organized existing elements and placed them on the periodic table according to atomic mass.

22. Henry Moseley 1887 - 1915 About 50 years later, Henry Moseley studied beta-emissions of all the elements and found relationships between the x-ray spectra of an element and its atomic number. They were unique for each element. He reorganized the periodic table by atomic number.

23. Modern Atomic Theory Quantum Mechanical Model: According to the modern atomic theory, electrons are arranged into ‘energy levels’. The chemical properties of elements depend on how many electrons are in the various energy levels and how the electrons are arranged within the energy levels.

24. Practice WS #0 – Atomic Theory VIP’s