A Call For Volunteers

1. A Call For Volunteers • I have been approached by the special needs department here at the university requesting a student interested in helping another student in class by taking notes. • Anyone interested in this should see me after class to make arrangements. Chemistry 140 Fall 2002 Dutton

2. What about a supersaturated solution? • Consider a supersaturated salt solution. • Because it is a mixture it is chemically heterogeneous (salt and water) • Because there is dissolved and undissolved salt in the beaker, there are two phases of the salt making it physically heterogeneous (two different phases). Chemistry 140 Fall 2002 Dutton

3. General Chemistry Principles and Modern Applications Petrucci • Harwood • Herring 8th Edition Chapter 2: Atoms and the Atomic Theory Chemistry 140 Fall 2002 Dutton

4. Contents • Early chemical discoveries • Electrons and the Nuclear Atom • Chemical Elements • Atomic Masses • The Mole (save this for Friday) Chemistry 140 Fall 2002 Dutton

5. Early Discoveries Antoine Lavoisier Law of conservation of mass Proust 1799 Law of constant composition Before reaction = 0.455 g Mg + 2.315 g O2 = 2.770 g After reaction = ? g MgO + 2.015 g O2 = 2.770 g 0.755 g MgO Chemistry 140 Fall 2002 Dutton

6. Early Discoveries Lavoisier 1774 Law of conservation of mass Proust 1799 Law of constant composition Before reaction = 0.455 g Mg + 2.315 g O2 = 2.770 g After reaction = ? g MgO + 2.015 g O2 = 2.770 g 0.755 g MgO Sample A Composition Sample B 10.000 g 27.000 g 1.119 g H % H = 11.19 3.021 g H 8.881 g O % O = 88.81 23.979 g O Chemistry 140 Fall 2002 Dutton

7. Dalton’s Atomic Theory (1803-1808) • Each element is composed of small particles called atoms. • Atoms are neither created nor destroyed in chemical reactions. • All atoms of a given element are identical. • Compounds are formed when atoms of more than one element combine. John Dalton (1766-1844) Chemistry 140 Fall 2002 Dutton

8. Consequences of Dalton’s theory • Law of Definite Proportions: combinations of elements are in ratios of small whole numbers. • In forming carbon monoxide, 1.33 g of oxygen combines with 1.0 g of carbon. • In the formation of carbon dioxide 2.66 g of oxygen combines with 1.0 g of carbon. Chemistry 140 Fall 2002 Dutton

9. Behavior of charges Like charges repel each other Opposite charges attract Oppositely charged particles are deflected in opposite directions by a magnetic field. Chemistry 140 Fall 2002 Dutton

10. Cathode ray tube (Discovery of Electrons) Chemistry 140 Fall 2002 Dutton

11. Properties of cathode rays Electric Field Magnetic Field Electron m/e = -5.6857 x 10-9 g coulomb-1 Chemistry 140 Fall 2002 Dutton

12. Charge on the electron • From 1906-1914 Robert Millikan showed ionized oil drops can be balanced against the pull of gravity by an electric field. • The charge is an integral multiple of the electronic charge, e. Chemistry 140 Fall 2002 Dutton

13. Radioactivity Radioactivity is the spontaneous emission of radiation from a substance. • X-rays and g-rays are high-energy light. • a-particles are a stream of helium nuclei, He2+. • b-particles are a stream of high speed electrons that originate in the nucleus. Chemistry 140 Fall 2002 Dutton

14. The nuclear atom Geiger and Rutherford1909 To study Thompson’s model of the atom • Majority of α-particles penetrated undeflected. • Some α-particles experienced slight deflections. • A few (1/20000) suffered serious deflections. • A few didn’t penetrate but deflected back from where they came. Chemistry 140 Fall 2002 Dutton

15. The a-particle experiment • Most of the mass and all of the positive charge is concentrated in a small region called the nucleus . What actually happened • There are as many electrons outside the nucleus as there are units of positive charge on the nucleus What was expected Chemistry 140 Fall 2002 Dutton

16. The nuclear atom Rutherfordprotons 1919 • Most of the mass and all of the positive charge are centered in a very small region called the nucleus. • The magnitude of the positive charge is different for different atoms and is ½ of the atomic weight. • The atom as a whole is electrically neutral. Protons = electrons. James Chadwick neutrons 1932 Chemistry 140 Fall 2002 Dutton

17. Nuclear Structure Atomic Diameter 10-8 cmNuclear diameter 10-13 cm 1 Å Particle Mass Charge kg (SI) amu Coulombs (SI) (e) Electron 9.109 x 10-31 0.000548 –1.602 x 10-19 –1 Proton 1.673 x 10-27 1.00073 +1.602 x 10-19+1 Neutron 1.675 x 10-27 1.00087 0 0 Chemistry 140 Fall 2002 Dutton

18. Scale of Atoms • The heaviest atom has a mass of only 4.8 x 10-22 g • and a diameter of only 5 x 10-10 m. • Useful units: • 1 amu (atomic mass unit) = 1.66054 x 10-24 kg • 1 pm (picometer) = 1 x 10-12 m • 1 Å (Angstrom) = 1 x 10-10 m = 100 pm = 1 x 10-8 cm Biggest atom is 240 amu and is 50 Å across. Typical C-C bond length 154 pm (1.54 Å) Molecular models are 1 Å /inch or about 0.4 Å /cm Chemistry 140 Fall 2002 Dutton

19. Isotopes, atomic numbers and mass numbers • To represent a particular atom we use the symbolism: A= mass number Z = atomic number Chemistry 140 Fall 2002 Dutton

20. Measuring atomic masses Chemistry 140 Fall 2002 Dutton