CHEMISTRY 161 Chapter 2

1. CHEMISTRY 161 Chapter 2

2. 2.1. The Early Atomic Theory pure substances combine different elements compounds elements H2O [O3] H2, O2 John Dalton (1808) matter is composed of ‘building blocks’ ATOMS

3. Dalton’s postulates • Elements are composed of small particles called atoms; • atoms of an element are identical with same properties (mass) • 2. Compounds are composed of atoms of more than one element; • in a compound, the ratio of the number of atoms is an integer or fraction • law of definite proportions (Joseph Prost) • 3. A chemical reaction involves separation and combination of atoms; • atoms are neither created nor destroyed an atom is the basic, undividable unit of any element

4. The Scientific Method 1. experiment 2. data collection 3. data interpretation 4. hypothesis

5. 2.2. Structure of an Atom subatomic particles electrons protons neutrons 1897 - 1932

6. 2.2.1. The Electron Joseph Thomson (1897)

7. Cathode Ray Tube cathode (-) Fluorescent screen anode (+) focus anode (+)

8. Deflection of Cathode Rays + - - + anode (+) fluorescent screen cathode (-) focus anode (+) • particles are negatively charged; particles are defined as ‘electrons’ • 2. charge to mass of a single electron -1.76 × 105 C/Kg

9. Charge of Electrons charge of an electron = -1.6022 × 10-19 C mass of an electron = 9.10 × 10-31 kg

10. 2.2.2. The Proton 1. atoms contain electrons 2. atoms are electrically neutral positive charges must exist in an atom

11. Discovery of Radioactivity - lead block + radioactive substance

12. Production of three rays 1. Alpha () rays consist of positively charged particles  particles 2. Beta () rays consist of negatively charged particles electrons 3. Gamma () rays consist of high energy rays no charge

13. Ernest Rutherford

14. gold foil  particle emitter slit detection screen 1. most  particles penetrate the gold foil 2. a few  particles are deflected 3. a very few  particles bounce back

15. Rutherford Model of the Atom 1. most of the atom space is empty 2. the atom’s positive charge is located in a small, dense central core ‘nucleus’ (repulsive force between positively charged particles) 3. electrons are surrounding the nucleus to neutralize the charge - +

16. Canal Rays glowing cathode with holes anode positive ion discharge of hydrogen gas (H2) deflection of positive particles in a field

17. proton 1.67262 × 10-27kg +1.6022 × 10-19C +1 electron 9.10939 × 10-31kg -1.6022 × 10-19C -1 m(p+) / m(e-) ≈ 1840 Rutherford the atom’s positive charge is located in a small, dense central core ‘nucleus’ protons are a constituent of the nucleus

18. + + + + + 1. mass of the nucleus constitutes most of the mass of the atom 2. the nucleus is positively charged and contains protons 3. the nucleus constitutes only about 1/1013 space of an atom atom radius ≈ 100 pm nucleus radius ≈ 5 ×10-3 pm 4. electrons are as clouds around the nucleus

19. 2.2.3. The Neutron nucleus expected nucleus observed nucleus charge mass [p+] mass [p+] H atom + 1 1 1 He atom + 2 2 ≈ 4 Where is the missing mass?

20. high energy rays  particles generate high energy rays upon interaction with beryllium which cannot be deflected by magnetic field James Chadwick (1932)

21. Beryllium Paraffine (CxHy) counter 1. high energy rays have no charge and are neutral 2. high energy rays contain particles which have a mass slightly more than the proton NEUTRONS 3. neutrons help to equilibrate the mass balance the nucleus

22. electrons protons nucleus mass [p+] H atom 1 1 1 He atom 2 2 ≈ 4 helium nucleus has two protons and two neutrons

23. Summary proton, p 1.67262 × 10-27kg +1.6022 × 10-19C +1 electron, e 9.10939 × 10-31kg -1.6022 × 10-19C -1 neutron, n 1.67493 × 10-27kg 0 0

24. Homework Chapter 2, p. 35-45 problems