Atomic history

1. Atomic history

2. Dalton's Atomic Theory (1) Matter is made of atoms; atoms are indivisible and indestructible. (2) All atoms of one element are exactly identical to one another; atoms of different elements are different from one another. (3) Atoms are neither created nor destroyed in chemical reactions. (4)Atoms combine to form compounds. A given compound always has the same relative number and kind of atoms. (5) Chemical reactions are needed to rearrange compounds.

3. INSTANT RESPONSE Which one of the following is not one of the postulates of Dalton's atomic theory? A. Atoms are made of protons, neutrons, and electrons. B . All atoms of a given element are identical C . Atoms are neither created nor destroyed in chemical reactions. D . Compounds are formed when atoms of more than one element combine E . Each element is composed of extremely small particles called atoms.

4. Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle: the electron http://www.youtube.com/watch?v=O9Goyscbazk&feature=related • Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

5. INSTANT RESPONSE Which one of the following is not true concerning cathode rays? A. They originate from the negative electrode. B. They travel in straight lines in the absence of electric or magnetic fields. C. They impart a negative charge to metals exposed to them. D. They are made up of electrons. E. The characteristics of cathode rays depend on the material from which they are emitted.

6. JJ Thompson’s Idea • He knew that the atom contained + and – charges. • The atom was like “Plum Pudding” Electrons scattered in a cloud of positively charged matter

7. Mass of the Electron Mass of the electron is 9.11 x 10-28 g The oil drop apparatus 1916 – Robert Millikan determines the mass of the electron: 1/1840 the mass of a hydrogen atom; has one unit of negative charge

8. Discovery and Properties of the Electron q was always an integer multiple of the same number, which was given the symbol "e" The currently accepted value of e is: 1.602x10-19C Knowing e allows the electron mass to be calculated: 9.11 X 10 -31 kg electrons x integers = charge

9. INSTANT RESPONSE Which of these could be the charge of an object? A. 0.80 x 10-19 C B. 2.0 x 10-19 C C. 3.2 x 10-19 C D. 4.0 x 10-19 C

10. INSTANT RESPONSE The magnitude of the charge on an electron was determined in the __________. A. cathode ray tube, by J. J. Thomson B. Millikan oil drop experiment C. Dalton atomic theory D. atomic theory of matter

11. Conclusions from the Study of the Electron: Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons. Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons Electrons have so little mass that atoms must contain other particles that account for most of the mass

12. Ernest Rutherford’sGold Foil Experiment - 1911 • Alpha particles are helium nuclei - The alpha particles were fired at a thin sheet of gold foil • Particles that hit on the detecting screen (film) are recorded

13. Rutherford’s Experiment Gold Foil • Shot a particles at gold. • Most pass through. • Very few deflected • Giegor and Marsden were scientists who worked under Rutherford to carry out the experiment

14. Rutherford’s Findings • Most of the particles passed right through • A few particles were deflected • VERY FEW were greatly deflected Conclusions: The nucleus is small The nucleus is dense The nucleus is positively charged The atom is mostly empty space

15. INSTANT RESPONSE The gold foil experiment performed in Rutherford's lab __________. A. confirmed the plum-pudding model of the atom B . led to the discovery of the atomic nucleus C. was the basis for Thomson's model of the atom D. utilized the deflection of beta particles by gold foil E. proved the law of multiple proportions

16. INSTANT RESPONSE In the Rutherford nuclear-atom model: A. the heavy subatomic particles reside in the nucleus B. the principal subatomic particles all have essentially the same mass C. the light subatomic particles reside in the nucleus D. mass is spread essentially uniformly throughout the atom

17. James Chadwick Studied the mass of the nucleus of the atom and realized there is more mass than number of protons. The neutron accounted for the extra mass. It was so hard to find because it is neutral and in the nucleus.

18. The Bohr Model • Studied the light emitted from atoms when excited by electricity - Spectroscopy • Add Energy – electrons jump up to a higher level. • Remove Energy – electrons fall back to lower level.

20. Niels Bohr 1913 • Electrons exist on energy levels. • Like planets orbiting the sun • Each orbit only contains a set number of electrons. • Each orbit possessed a given quantum of energy.

21. Bohr’s Atom electrons in orbits nucleus

22. The electron cloud • The area in space surrounding the nucleus that has a high probability of holding an electron. • Schrodinger developed the cloud model.

23. Schrodinger’s Modern Model of the Atom • Atom still has a positive nucleus. • Electron location based on probability. • Created cloud shapes to show the most probable location of an electron.

24. HELIUM ATOM Shell proton N + - + N - neutron electron

25. ATOMIC STRUCTURE Particle Charge Mass proton + charge 1 neutron No charge 1 electron - charge nil