1. History of the Atom

2. Aristotle 400 BC believed there were four elements Earth, Wind, Fire and Water

4. Democritus 460 BC Believed that all matter consisted of small particles that could not be divided If you cut something in half again and again eventually you would reach a point that couldn’t be cut Atomos—greek for indivisible, point which cannot be cut in half.

5. Robert Boyle 1600’s Father of Modern Chemistry All science should be experimentally based Developed Boyle’s Law which dealt with pressure and volume of gasses.

6. Dalton 1803 English scientist Measured masses of elements when combining as compounds Determined that ratios of elements in compound are always the same

7. More of Dalton! All elements are composed of atoms All atoms of the same element have the same mass, and different elements have different masses Compounds contain atoms of more than one element Atoms of different elements always combine in the same way in a particular compound

8. Jon Berzilius 1814 Developed modern chemical formula notation

9. J J Thomson Discovered that atoms have positive and negative charges Used glass tube with positive end and negative end, beam of light went through it. Beam deflected toward positive plate Electrons negative

10. Positive plate Thompson’s Tube

11. Thomson’s atomic model Plum pudding model Plum pudding is a fruit cake, the cake is the positive part with the fruit bits being the negative particles

12. Rutherford Discovered that uranium emits fast moving positive charge Used positive particles (alpha particles) to bombard gold foil

13. Gold Foil Experiment

14. As alpha particles passed through, most went straight through foil. Some veered off to one side Very few bounced straight back Theorized nucleus as small densely packed positive central area

15. Timeline Democritus 400 BC atomos, indivisible Aristotle 300 BC earth fire wind and water Dalton 1766 atoms made of elements, compounds made of fixed ratios of different elements Thomson late 1800’s negative and positive particles in atom Rutherford 1899 gold foil experiment, discovered positively charged nucleus

16. Properties of subatomic particles ParticleSymbolChargeMass in a.m.u.

17. Protons positively charged subatomic particle found in the nucleus of an atom. charge of 1+ Mass of 1 a.m.u.

18. Neutron neutral subatomic particle found in the nucleus mass 1 a.m.u.

19. Electron negatively charged subatomic particle found in the space outside the nucleus (electron cloud) charge of 1− So small virtually no mass

21. Bohr 1913 electrons move with constant speed in fixed orbits around the nucleus, like planets around a sun. energy levels: The possible energies that electrons in an atom can have An electron in an atom can move from one energy level to another when the atom gains or loses energy.

22. Electron Cloud Scientists use the electron cloud model to describe the possible locations of electrons around the nucleus. First energy level: can hold 2 electrons Second energy level: can hold 8 electrons Third energy level: can hold 8 electrons** Fourth energy level: can hold 18 electrons**

23. Atomic Number equals the number of protons in an atom of that element. Atoms of different elements have different numbers of protons

24. Mass Number the sum of the protons and neutrons in the nucleus of that atom. Mass number = neutrons + protons We write mass number as a superscript to left of symbol

25. Isotopes have the same atomic number different mass numbers because they have different numbers of neutrons. Value on periodic table is an average of the masses of the isotopes

26. Atomic Mass Mass Number is protons + neutrons. Atomic Mass is an average of the mass numbers of different isotopes.