2. Chapter 3 Chemistry

3. The Atom I. Early ideas A. ___________ (450 BC)- all matter is made of tiny indivisible particles – atomos. _________- the smallest particle of an element that retains the properties of the element. B. ____________________- (late 1700’s) 1. Law of Conservation of Matter

4. Early Ideas Cont: C. __________- (1799) – Law of Constant Composition Atoms form cmpds in specific ratios determined by mass.

5. Early Ideas Cont: D. _________________ (1803) English 1. Atomic Theory of Matter a.) Each element is composed of atoms (Democritus) b.) All atoms of an element are the same; atoms of different elements are different c.) Atoms are not created or destroyed in rxns. (Lavoisier) d.) Cmpds have the same relative # and kinds of atoms. (Proust)

6. Early Ideas Cont: E. __________________ (late 1700’s) a) electric charges: like repel; opposites attract F. ____________(1839) - atoms contain particles with electric charges G. ____________________ (1870’s) – a. Crookes’ Tube exp.

7. Early Ideas Cont: H. _______(1896) – Cathode ray tubes I. _____________ (1896) – radioactivity J. ________________ (1909) – Oil Drop Exp. K. ___________________ (1909) – Gold Foil Exp. (Alpha scattering exp.)

8. II. Early Experiments A. ____________________ – key experiment 1. Results – two kinds of charge: + & - like charges repel; opp. attract B. Crookes’ Tube Experiment 1. Partially evacuated tube filled w/a low pressure gas. 2. 2 electrodes inside – anode (+) cathode (-)

9. Early Exp. Cont: 3. When voltage was applied, a green glow originated in the tube. a) produced a shadow at the end opposite the cathode;  it had to originate at the cathode CATHODE RAY; the tube was then called a CATHODE RAY TUBE. 4. Was it light or charged particles?? a) Beam was deflected by a magnet,  it had to be a stream of charged particles. www.chem.uidaho.edu/~honors/crookes.html http//library.thinkquest.org/28582/history/dalton.htm

10. Early Exp. Cont: C. Thomson’s Exp. 1. Build a CRT with a fluorescent screen at end opposite the cathode. Glowed where particles struck. 2. Added a magnet and a pair of charged plates. a) the beam was 1 st deflected by the magnet, but was attracted to the + charged plate; therefore the particles had to have a – charge.

11. Early Exp. Cont: 3. Changing the electrodes and the gases used always yielded the same results;  the particles were identical and were found in all atoms. These particles were called _______________________.

12. Early Exp. Cont: D. The PROTON 1. If atoms had – particles, they also had to have + particles, because atoms were electrically neutral. 2. Discovered through exp. w/CRTs. 3. 1885- Thomson used H gas in a CRT. When voltage was applied, the ray moved to the cathode (-); therefore it had to be positive.

13. Early Exp. Cont: 4. The amount of deflection varied with the different gases. 5. _____________________- electrons were randomly distributed throughout a + charged “gel”.

14. Early Exp. Cont: E. Becquerel’s discovery- __________ 1. Placed a piece of uranium on a photographic plate and placed it in a dark drawer. 2. Later found the plate had been exposed by some unknown type of radiation originating from the U.

15. Early Exp. Cont: F. Ernest Rutherford- radioactivity 1. Through exp. determined the 3 types of radiation G. Ernest Rutherford (1909)- 1. Gold Foil Exp. (alpha-scattering Exp.) a) Aimed a beam of high speed α particles (+) at a very thin piece of gold foil. b) Almost all particles passed through

16. Early Exp. Cont: c) Some particles had slight deflections d) Very few particles reversed direction and came straight back. e) Results: All + charge is concentrated at the center of the atom – __________________. 1. Credited with the ___________________________ of the ATOM.

17. III. Atomic Structure A. Nucleus 1. + Charged 2. Composed of p + (+) and n (no chg) B. Electron Cloud 1. e - travel around nucleus in discrete energy levels 2. In a neutral atom, the # e - = the # p +

18. IV. Atomic Numbers A. Discovered by Henry Moseley 1. Atoms of @ element have a unique + chg. on the nucleus B. The identity of the element depends on the # p + in nucleus ATOMIC NUMBER

19. V. Ions A. Ions- an electrically charged atom B. Formed when atoms gain or lose e - ; or share e - C. ATOMS NEVER LOSE PROTONS D. If gain e - = - chg; If lose e - = + chg

20. Isotopes ________________ – atoms of the same element with the same number of protons but different numbers of neutrons Ex: H-1, H-2, H-2 C-12, C-13, C-14

21. Atomic Mass Atoms are too small to see individually They are also too small to mass with normal everyday lab balances So, how is the mass of a SINGLE atom measured????

22. Atomic Mass Atomic Mass Unit – the mass of a single proton or neutron Protons and neutrons have essentially the same mass – they make up 99% of the mass of the atom Therefore, if we add those numbers up we get the mass of a single atom

23. Atomic Mass Scientists chose to define an atomic mass unit in terms of an arbitrary standard – the C-12 atom 1 atom of C-12 is exactly 12 amu So, why are the atomic masses listed on the periodic table not whole numbers?

24. Atomic Mass _____________________ – the fractional occurrence in nature of a specific isotope of an element _________________________ – the weighted average of all the isotopes of an element.

25. Atomic Mass Example: