History of the Atom

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. Gold Foil Experiment • 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

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.