Development of the Atomic Model

1. Development of the Atomic Model

2. Who are these men? Men whose quests for knowledge about the fundamental nature of the universe helped define our views.

3. The Philosophers • Earliest ideas of matter • “all things were made of four elements: Earth, Air, Fire, and Water

4. The Philosophers • Democritus– Greek 400 B.C. • World made of 2 things: 1) Empty space 2) Indivisible particles called “ATOMS” • All matter made of different shapes, tiny, indivisible and indestructible particles

5. Atomos: Not to Be Cut

6. Atomos • To Democritus, atoms were small, hard particles that were all made of the same material but were differentshapes and sizes.

7. This theory was ignored and forgotten for more than 2000 years!

8. Why? • The philosophers of the time, Aristotleand Plato, had a more respected theory (andultimately wrong). Aristotle and Plato favored the earth,fire, air and water approach to the nature of matter. Their ideas were believed because of their role as philosophers. The atomos idea was buried for approximately 2000 years.

9. John Dalton (1766-1844) • In the1800s, the English Chemist John Daltonperformed a number of experiments that eventually led to the acceptance of the idea of atoms.

10. The atomic model has changed throughout the centuries, starting in 400 BC. John Dalton’s model is referred to as the “billiard ball” model. →

11. ProposedAtomic Theoryin 1803 • All (matter) elementswas composed of indivisible and indestructible particles termed atoms. • Atoms of the same element were exactly alike. • Atoms of different elements were not alike. • Compoundsare formed by the joining of atoms of two or more elements.

12. According toDalton’stheory, an element is composed of only one kind of atom, and a compound is composed of particles that are chemical combination of different kinds of atoms. How does a mixture differ from a compound?

13. Proposed Law of Multiple Proportions • Atoms of two or more elements may combine in different ratios to produce more than one compound. CO and CO2 EX. H2O & H2O2 water hydrogen peroxide H2O :Ratio of oxygen is 1:2 (an exact ratio)

14. This theory became one of the foundations of modern chemistry.

15. J.J.Thomson’s(1856 –1940)British • In 1897, the English scientist J.J. Thomson provided the first hint that an atom is made of even smallerparticles.

16. Thomson Model • Studied the passageof an electriccurrent through a gas. • As current passed through the gas, it gave off rays of negatively charged particles.

17. Cathode ray tube • Used a cathode ray tube to discover that atoms contained small, negatively charge particle • Particles named: ELECTRONS • (first subatomic particle to be found) • Hypothesized that atom had Positive (+) portions.

18. Plum Pudding model • He proposed a model of the atom that is called the “PlumPudding” model. • Atoms were made from a large cloud of positive particles within which negative electrons were randomly scattered. • All parts of the atom were lumped together and looked like pudding with raisins (electrons). An atom – sphere of positive charges containing electrons.

19. The Plum Pudding Model J.J. Thompson- 1904 Electrons (Negativly Charged) Field (Positively Charged) In Today's words- The Chocolate Chip Model

20. Robert Millikan (1911) American scientist who performed an OIL DROP EXPERIMENT and determined the mass and charge of an ELECTRON

21. Ernest Rutherford(1871 –1937) • New Zealand born, English physicist performed an experiment with gold foil.

22. Rutherford’s experiment Involved firing a stream of tiny positively chargedparticles at a thin sheet of gold foil (2000 atoms thick)

23. He concluded that an atom was made up of small, dense, positively chargednucleus • He invented name PROTON for positive part of atom

24. Rutherford • He also concluded that the atom was mostly empty space. • ( +) in nucleus) • The (–) were scattered outside the nucleus around the atom’s edge.

25. James Chadwich (1891 – 1974) • Discovered NEUTRON in 1932. • Neutron later used to initiate chain reaction in fission bombs (atomic bombs)

26. Neils Bohr (1885 –1962) • Born in Copenhagen and worked with Ernest Rutherford. • In 1913, In his model, he placed each electron in a specific energy level.

27. Bohr Model of Atom • Developed ;the BOHR Model • had a small, dense nucleus & electrons that orbited around nucleus, like planets orbit the sun.

28. Bohr Models • Nucleus is orbited by electrons, which are in different energy levels. • A model uses familiar ideas to explain unfamiliar facts observed in nature. • A model can be changed as new information is collected.

29. Wave Model

30. The Wave Model • Today’s atomic model is based on the principles of wavemechanics. • According to the theory of wave mechanics, electrons do not move about an atom in a definite path, like the planets around the sun.

31. Electron Cloud: • A space in which electrons are likely to be found. • Electrons whirl about the nucleus billions of times in one second • They are not moving around in definite patterns. • Location of electrons depends upon how much energy the electron has.