1. The History of the Atom…

2. went against, Aristotle, who believed that matter was composed of four qualities: earth, fire, air and water all matter is composed of small, indivisible particles coined the term ‘atomos’ John Dalton 1766-1844 developed a model on the atom based on already established laws: the law of conservation of matter (Lavoisier) the law of definite proportions (Proust) the law of multiple proportions (Dalton) Democritus 460 BC - 370 BC

3. Law of Conservation of Matter Law of Definite Proportions Law of Multiple Proportions Matter is neither created nor destroyed in a chemical reaction Elements combine in definite, fixed ratios to form compounds Example - water is H 2 O every time. - The same elements can combine in different fixed ratios to form different compounds. Example H 2 O versus H 2 O 2. - When 2 or more elements combine to form different compounds, the ratio of the second element to a fixed mass of the first is always a small, whole number ratio

4. 1) Each element is made up of indivisible particles of uniform density - atoms. (now know atoms are divisible) 2) All atoms of a given element have the same mass, atoms of different elements have different masses. (now know about isotopes) 3) Atoms combine in fixed, small, whole number ratios to form compounds. (explains law of definite and law of multiple proportions) 4)Chemical reactions involve the rearrangement of atoms to form new substances. (atoms are neither created nor destroyed) (explains law of conservation of mass) Dalton’s model of the atom: Dalton’s Atomic Theory 1803

5. Crookes and the Cathode Ray Tube (1870’s) Particles or light?? Test with a magnet:

6. What were these particles??? Thomson’ hypothesis: since the beam came from the cathode (a negative electrode), and given how the ray is deflected by a magnet, the particles have a negative charge. Studied the deflection of the cathode ray by an electric field to determine if this was the case: J. J. Thomson 1856 - 1940

7. Tried to separate the charge from the particles by bending the rays with a magnet. He could not separate the two. Calculated the charge to mass ratio of these particles by measuring the degree of deflection with different strength magnetic and electric fields. no matter which gas he used in the tube, the calculated charge to mass ratio of the particles was the same. AP disk chap 2 screen 6 description

8. But atoms are electrically neutral - must also have a positive component. Reverse the electrodes on the cathode ray tube: deflection by positive plate!! More J. J. Thomson Voila! A positive component. Thomson used the hydrogen ion, which is just a single proton, so he missed that atoms have positively charges particles - he thought the whole atom was a ball of positive charge.

9. Thomson concluded that the cathode ray was made up of negatively charged subatomic particles, later called electrons (e - ) No matter what metal the electrodes were made of, or what gas filled the tube, the same charge to mass ratio was calculated. All atoms have them, and they are identical in all atoms Atoms also have a positive component - he reasoned a ball of positive charge, protons were found later. ball o’ positive charge electrons Thomson’s Plum Pudding Model (1896ish)

10. Millikan’s Oil Drop Experiment 1909 Determined the charge of an electron. Given Thomson’s charge to mass ratio calculation, the mass of an electron could then also be determined. Oil drops were given a negative charge by attaching electrons to them. By balancing the force of gravity with the force from an electric field on these droplets, Millikan could determine the magnitude of the charge on each oil drop, and found that the charge was always some multiple of 1.6 x 10 -19 C. Therefore, the charge of a single electron was 1.6 x 10 -19 C. AP disk chapter 2 screen 7 description for animation

11. Was trying to prove the Plum Pudding model with his famous Gold Foil Experiment in 1909 expected most of the (+)ly charged alpha particles to pass straight through the gold atoms in the foil. Ernest Rutherford 1871 - 1937

12. Most of the alpha particles passed straight through, as expected. Some were deflected at large, large angles - some came right back at him!! This was unexpected…. AP disk chapter 2 screen 9 description Rutherford’s Observations What was going on???

13. Rutherford’s Conclusions and Model Majority of the atom was empty space, with all the positive charge and 99% of the atom’s mass concentrated in a small volume in the center. The nucleus!! (small and dense) Electrons orbited around the nucleus. Much smaller than protons. Two problems: 1. The mass of atom did not equal the mass of the positive charge alone. 2. Why didn’t the electrons fall into the nucleus?

14. Rutherford’s model suggested atoms composed of two subatomic particles with equal but opposite charges: electrons and protons. Problem - mass of atom not equal to mass of protons alone - must be another subatomic particle to make up the difference. 1932 - Chadwick discovers the neutron - a subatomic particle present in all atoms that has about the same mass as a proton but no electrical charge!! Rutherford’s First Problem