1. S. Aldrich 2008

2. HISTORY OF THE ATOM 460 BCE Democritus develops the idea of atoms He believed that you could cut objects in half, over and over again, until you would end up with a particle that could not be cut, that he called ATOMOS (Greek word meaning “indivisible” or “not able to be divided”)

3. HISTORY OF THE ATOM 1808 John Dalton (atoms combine to form compounds ) He suggested that all substances were made up of atoms and that atoms could not be created, divided or destroyed. Atoms of the same element are exactly the same and atoms of different elements are different in mass and size.. Atoms join with other atoms to make new substances.

4. HISTORY OF THE ATOM 1897 Joseph John (JJ) Thompson (discovered the 1 st subatomic particles) He discovered that there are small particles inside the atom, so atoms could be divided into smaller parts. He conducted the cathode ray tube experiment and found that atoms could sometimes eject a far smaller negative particle which he called We now call them ELECTRONS corpuscles

6. Thompson develops the idea that an atom was made up of electrons scattered unevenly within a sphere surrounded by a positive material to balance the electron's charge like plums surrounded by pudding. In this model the plums represent the electrons and the pudding represents the positively charged material. 1904 PLUM PUDDING MODEL

7. HISTORY OF THE ATOM 1911 Ernest Rutherford (discovered the nucleus) He conducted an experiment to study the structure of an atom. He fired positively charged particles at a piece of gold foil which was only a few atoms thick. He found that although most of them passed straight 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

10. HISTORY OF THE ATOM Rutherford’s new evidence allowed him to propose a more detailed model with a central nucleus that had a positive charge. He suggested electrons travel in the empty space outside of the nucleus, but that their exact paths could not be described. However, this was not the end of the story...

11. HISTORY OF THE ATOM 1913 Niels Bohr (discovered energy levels) He studied under Rutherford at the Victoria University in Manchester. Bohr refined Rutherford's idea by adding that the electrons were in orbits traveling around the nucleus in definite paths- with each orbit only able to contain a certain number of electrons.

12. Bohr’s Atom electrons in orbits nucleus

13. HISTORY OF THE ATOM 1924 Erwin Schrodinger He developed the Electron Cloud model, proposing that the exact path of electrons cannot be predicted. He believed that the electron cloud was the area where electrons were likely to be found.

14. HISTORY OF THE ATOM 1932 James Chadwick (discovered the neutron) 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). Chadwick won the Nobel Prize in 1935. Interestingly, the discovery of the neutron led directly to the discovery of fission (the splitting of atoms) and ultimately to nuclear energy, nuclear explosions and the atomic bomb.

15. HISTORY OF THE ATOM Modern Theory

16. HELIUM ATOM + 0 0 + - - proton electron neutron Electron cloud, electron shell or electron orbital What are the properties of these particles? Nucleus HISTORY OF THE ATOM

17. ATOMIC STRUCTURE Subatomic Particle proton neutron electron Charge +positive charge -negative charge No charge 1 amu Almost zero Mass Location Inside the nucleus Outside the nucleus; in electron clouds The SI unit for the masses of particles in atoms is the atomic mass unit (amu)

18. So, where is most of the mass of the atom located? Subatomic Particle proton neutron electron Charge +positive charge -negative charge No charge 1 amu Almost zero Mass Location Inside the nucleus Outside the nucleus; in electron clouds

19. ATOMIC STRUCTURE the number of protons in an atom the number of protons + neutrons in an atom (add up everything in the nucleus) He 2 4.003 Atomic mass = Atomic number = For neutral or stable atoms: number of electrons = number of protons helium **the type of atom is determined by the number of protons it has**

20. To figure out the number of neutrons: Take the atomic mass and round it to a whole number. From that, subtract the number of protons.

21. Element Number of Protons Number of Neutrons Number of Electrons Atomic Mass Atomic Number lithium carbon chlorine silver lead calcium 3 6.941 334 666 12.011 6 17 35.453 18 47 107.868 61 82 207.2 125 20 40.078 20