History of The Atom

1. History of The Atom

2. Democritus – 400B.C. • He believed matter was made up of empty space and tiny particles he called “atomos” • Said different things were made with different types of atoms.

3. Aristotle – 335 B.C. • Matter was continuous – “what you see is what you get” • Identified four elements – earth, wind, fire, and water

4. Galileo – Late 1500’s • First person to make extensive use of the experimental method to study natural phenomena • First to agree with Aristotle • Agreed with Copernicus that sun was center of the universe.

5. Sir Isaac Newton – Late 1600’s • English Scientist • When he was 23-24 years old he was stuck at home during the Bubonic Plague. At this time he invented the following: • Calculus • Laws of Motion • Laws of Gravity • Did lots of studies of lights • The atom idea resurfaced- he said that there were small particles but he offered no proof.

6. Antoine Lavoisier -1789 • Law of Conservation of Mass • Matter cannot be created or destroyed!!!!!

7. Joseph Proust - 1799 • Law of Definite Proportions • Compounds made of the same elements in the same ratios!!

8. John Dalton - 1803 • Published a series of papers on the findings of Proust and Lavoisier. • Dalton’s atomic theory • 1. Elements are made of tiny particles called atoms. • 2. All atoms of a given element are identical. • 3. The atoms of a given element are different from those of any other element; the atoms of different elements can be distinguished from one another by their respective relative weights. • 4. Atoms of one element can combine the atoms with atoms of other elements to form chemical compounds; a given compound always has the same relative number of types of atoms. • 5. Atoms cannot be created, divided into smaller particles, nor destroyed in the chemical process; a chemical reaction simply changes the way atoms are grouped together.

9. John Dalton - 1803 • Law of Multiple Proportions • Dalton’s model of the atoms – he used solid wooden balls to show matter was made of indivisible particles.

10. Eugene Goldstein - 1876 • Introduce the term cathode ray and its positive counterpart the canal ray.

11. Wilhelm Roentgen - 1895 • Discovered x-rays

12. Antoine Henry Becqueral - 1896 • Discovered Radioactivity • The phenomena of the unstable nuclei of certain elements spontaneously emitting particles of energy.

13. Antoine Henry Becqueral - 1896 C. Forms of radiation • Alpha particle (α)[Rutherford] • Helium nucleus (2 neutrons and 2 protons) • Do not penetrate solids • Very large, has a + charge • Because of high KE, they can damage surface tissue

14. Antoine Henry Becqueral- 1896 • Beta Particle (β) [Rutherford] • High speed electron is formed at the moment of decay • Hard to stop • Penetrates light materials; clothing, paper, etc. • Cannot penetrate most metals

15. Antoine Henry Becqueral- 1896 • Gamma Ray ( γ) [Villard] • High energy x-rays • Have no (-) charge and no mass • Pure energy • Can penetrate through most materials (metals, walls, etc.) • Cannot penetrate through dense materials like lead.

16. JJ Thomson - 1897 • Did research on cathode rays and built a (CRT) or cathode ray tube [we us these tubes in TV’s and computer screens] • A CRT is a glass tube filled with a gas with 2 electrodes attached (anode = positive electrode and cathode = negative electrode) • Passed a voltage between the electrodes, saw a light beam which he called a cathode ray. • Subjected the beam to electrical and magnetic fields, found the beam to be negatively charged (he called the particles electrons)

17. JJ Thomson - 1897

18. JJ Thomson - 1897 • HIS MODEL OF THE ATOM • “Plum Pudding Model” • Negative particles with a positive core.

19. Robert Millikan -1911 • American physicist • Did the “Oil Drop Experiment” • In this he had oil drops that were charged pass through charged plates, he offset the voltage on the plates to get the oil drop to suspend briefly and wrote down the voltage. • He then found the exact charge on an electron to be -1 • He also found the mass of the electron to be 1/1897th of a hydrogen atom.

20. Lord Ernest Rutherford • A New Zealand Physicist • In 1911-1912 worked with Bohr, Geiger, and Marsden on the gold foil experiment.

21. Lord Ernest Rutherford C. Gold Foil Experiment Set-up • Lead box with pin hole • Put a piece of radioactive material inside which gave off alpha particles (positive particles) • Used a very thin sheet of gold foil as a target • Surrounded the foil with a screen coated in zinc sulfide that sparked when alpha particles hit it

22. Lord Ernest Rutherford

23. Lord Ernest Rutherford • Observations he made: • 99% of the particles went straight thru the gold foil like it wasn’t even there. • 0.9% went thru but passed thru at a slight angle • 0.1% hit the gold foil like a brick wall and bounced right back.

24. Lord Ernest Rutherford 2. Conclusions • Atoms made up mostly of empty space • Something must repel the positive alpha particles – must be a positively charged nucleus • All the mass of the atom is concentrated in one place- the nucleus which is surrounded by empty space • Stated that the + charged nucleus must be (+1) to offset the (-1) of the electron. (Atoms are neutral)

25. Lord Ernest Rutherford d. Rutherford’s model of the atom 1. Electrons travel in space around nucleus like the planets around the sun.

26. Neils Bohr Model -1913 a. Worked on gold foil

27. 1914 Discovery of the Proton • Thomson is working with CRT again and finds a positive particle that is larger and the exact opposite charge of the electron. • 1800 times heavier than the electron

28. Henry Moseley -1913 • Studied x-rays produced by different metals (used the metals as anodes) • Saw that each metal produced a different wavelength because each had a different number of protons • From this development the atomic number • Tells the number of protons • # of protons = # of electrons • # of protons determine the identity of the element

29. James Chadwick - 1932 • Found high energy particles with no charge and a mass equal to a proton. • Called them neutrons • Nucleons – particles found in the nucleus of an atom, also known as hadrons; these particles are protons and neutrons which are each made up of three quarks.

30. Mass Number • Mass # = # of protons + # of neutrons • # of neutrons = mass # - atomic # Ex. Carbon Mass #= 12 Atomic # = 6 # protons =# e- = 6 # of neutrons = 6

31. #protons =86 # electrons = 86 #neutrons = 136

32. Isotopes • JJ Thomson discovered • Atoms of the same element that have a different mass due to the fact that have different # of neutrons • Disproves Dalton’s theory that atoms of the same element are the same

33. Average Atomic Mass (# on the periodic table) • Scientists found the masses of all the isotopes and their percent abundance with a mass spectrometer • A mass spectrometer compares each element to Carbon-12 atom • Measured in atomic mass unit (amu) • The average atomic mass was calculated using those numbers (get a decimal #, not a whole #, because it’s an average)

34. Ex. Find average atomic mass (from isotopes) Cu-63 62.930 amu=69.17% in the sample Cu-65 64.928 amu=30.83% in the sample

35. Find mass contribution of each isotope (mass) x ( %) Cu-63  62.930 x .6917 = 43.53 Cu–65  64.928 x .3083 = 20.02 • Find average mass by adding the isotopes mass contributions together Average atomic mass = 43.53 + 20.02 = 63.55 amu