The document discusses the history of the atomic model from ancient Greek philosophers to modern theory. Democritus first proposed the idea of atoms in 460 BC. In the early 1900s, experiments by Thomson, Rutherford, and Bohr led to developments in the atomic model. Rutherford discovered the nucleus at the center of the atom in 1910. Bohr then proposed his atomic model in 1913 where electrons orbit the nucleus in defined shells. Later models like Schrodinger's probability cloud refined and improved upon understanding of atomic structure. The modern view is that the atom is mostly empty space with a dense nucleus at the center and electron cloud region.

1. ATOMIC STRUCTURE S.MORRIS 2006 More free powerpoints at www.worldofteaching.com

2. HISTORY OF THE ATOM 460 BC Democritus develops the idea of atoms he pounded up materials in his pestle and mortar until he had reduced them to smaller and smaller particles which he called ATOMA ( greek for indivisible )

3. HISTORY OF THE ATOM 1808 John Dalton suggested that all matter was made up of tiny spheres that were able to bounce around with perfect elasticity and called them ATOMS

4. Dalton’s Atomic Theory All matter is made of tiny indivisible particles called atoms. Atoms of the same element are identical, those of different atoms are different. Atoms of different elements combine in whole number ratios to form compounds . Chemical reactions involve the rearrangement of atoms. 5 Atoms cannot be created or destroyed.

5. HISTORY OF THE ATOM 1898 Joseph John Thompson found that atoms could sometimes eject a far smaller negative particle which he called an ELECTRON

6. HISTORY OF THE ATOM Thompson develops the idea that an atom was made up of electrons scattered unevenly within an elastic sphere surrounded by a soup of positive charge to balance the electron's charge 1904 like plums surrounded by pudding. PLUM PUDDING MODEL

7. HISTORY OF THE ATOM 1910 Ernest Rutherford oversaw Geiger and Marsden carrying out his famous experiment. they fired Helium nuclei at a piece of gold foil which was only a few atoms thick. they found that although most of them passed through. About 1 in 10,000 hit

8. HISTORY OF THE ATOM gold foil helium nuclei They found that while most of the helium nuclei passed through the foil, a small number were deflected and, to their surprise, some helium nuclei bounced straight back. helium nuclei

9. HISTORY OF THE ATOM Rutherford’s new evidence allowed him to propose a more detailed model with a central nucleus . He suggested that the positive charge was all in a central nucleus. With this holding the electrons in place by electrical attraction However, this was not the end of the story.

10. HISTORY OF THE ATOM 1913 Niels Bohr studied under Rutherford at the Victoria University in Manchester. Bohr refined Rutherford's idea by adding that the electrons were in orbits . Rather like planets orbiting the sun. With each orbit only able to contain a set number of electrons.

11. Bohr Model of the Atom Proposed by Niels Bohr in 1915; Not completely correct, but has many features that are approximately correct. Overcame major flaw with Rutherford model, which predicted that an orbiting electron would emit white as it accelerated towards the nucleus. Gives us a basic conceptual model of electrons orbits and energies. Key success was in explaining the spectral emission lines of atomic hydrogen

12. Shortcomings of the Bohr Model 1. Treats each electron as a miniature planet, with definite radius and momentum - in direct violation of the uncertainty principle which dictates that position and momentum cannot be simultaneously determined. 2. It fails to provide any understanding of why certain spectral lines are brighter than others.

13. Schrodinger’s Cloud Model Schrödinger developed the ‘probability function’ for the hydrogen atom (and a few others). This describes a cloud-like region where the electron is likely to be found. It can not say with any certainty, where the electron actually is, yet can describe where it ought to be. ‘ Clarity through fuzzines’ is one way to describe the idea. The probable locations of the electron predicted by Schrödinger's equation happen to coincide with the locations specified in Bohr's model.

14. Modern View - Summary The atom is mostly empty space. Two regions. Nucleus- protons and neutrons. Electron cloud- region where you might find an electron.

15. HELIUM ATOM + N N + - - proton electron neutron Shell What do these particles consist of?

16. ATOMIC STRUCTURE Particle proton neutron electron Charge + ve charge -ve charge No charge 1 1 nil Mass

17. ATOMIC STRUCTURE the number of protons in an atom the number of protons and neutrons in an atom He 2 4 Atomic mass Atomic number number of electrons = number of protons

18. Isotopes That is not the end of the story… Even atoms of the same element can have different numbers of neutrons. Such atoms have the same atomic number but… Different atomic mass . Called isotopes . X