Chapter 4

1. Chapter 4 Atomic Theory

2. Democritus First person to purpose that matter was not infinitely divisible “atomos”

3. Aristotle Rejected atomic theory did not believe in “nothingness” of space

4. John Dalton (1766-1844) • Modern atomic theory – 19th century • Conservation of mass – separation, combination, or rearrangement of atoms • Was all of his theory accurate? NO!!

5. Dalton’s Atomic Theory (1808) • All matter is composed of extremely small particles called atoms • Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties • Atoms cannot be subdivided, created, or destroyed • Atoms of different elements combine in simple whole-number ratios to form chemical compounds • In chemical reactions, atoms are combined, separated, or rearranged

6. Defining the Atom • The smallest particle of an element that still retains the properties of the element • How small is an atom? • World population: 6300000000 • Atoms in a penny: 29000000000000000000000 • Scanning tunneling microscope – allows individual atoms to be seen

7. Cathode Ray Tube • Discovered Cathode Ray Tube • Used Cathode Ray tube to discover Electron William Crookes JJ Thomson

8. Discovery of the Electron 1897:J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle. Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

9. Some ModernCathode Ray Tubes

10. Cathode Ray Results • Cathode ray deflected in a magnetic field, indicated charged particles • Deflected towards positively charged plate, indicating particles must have negative charge • Altering gas, altering material had no effect on results, so particles must be in all matter • Called…. ELECTRONS!!!! • First subatomic particles! • Meant Dalton was… wrong!!!

11. Mass and charge of the Electron • 1909 – Robert Millikan determines the mass of the electron. • Mass of the electron was much smaller than that of the hydrogen atom, the smallest known atom • Meant atoms were divisible into subatomic particles • Mass = 9.1 x 10-28 = 1/1840 mass of hydrogen • Charge = -1

12. Conclusions from the Study of the Electron • Electrons are negative. • 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

13. Thomson’s Plum Pudding Model • Atom breakable!! • Atom has structure • Electrons suspended in a positively charged electric field • must have positive charge to balance negative charge of electrons and make the atom neutral • mass of atom due to electrons • atom mostly “empty” space • compared size of electron to size of atom • Cookie dough model

14. Plum Pudding Model

15. Rutherford’s Gold Foil Experiment • Alpha particles are positively charged • Particles were fired at a thin sheet of gold foil • Particle hits on the detecting screen (film) are recorded

16. Rutherford’s Findings • Most of the particles passed right through • A few particles were deflected • GREATLY Deflected particles were repulsed by positive charge of nucleus Conclusions: Nuclear Model

17. Rutherford’s Nuclear Model • The atom contains a tiny dense center called the nucleus • the volume is about 1/10 trillionth the volume of the atom • The nucleus is essentially the entire mass of the atom • The nucleus is positively charged • the amount of positive charge of the nucleus balances the negative charge of the electrons • The electrons move around in the empty space of the atom surrounding the nucleus

18. Protons and neutrons • Rutherford - Protons • Subatomic particle in nucleus • protons :+1 charge (equal, opposite of electrons) • Chadwick – Neutrons • Subatomic particle in nucleus • Mass nearly equal to a proton, but carries no electrical charge

19. Atom are indivisible! What!?! Actually, Mr. Dalton, we have proved that part of your theory wrong. Atoms can be divided into electrons, protons, and neutrons.

20. The Structure of the Atom • Atom – electrically neutral particle composed of protons, neutrons, electrons • Spherical shape • Atoms consist of two regions • Nucleus – 99.7% of mass • Very small, dense region in the center. • Contains protons & neutrons. • Electrons Cloud • Mainly empty space surrounding nucleus • Very large compared to the nucleus. • Contains electrons. • Subatomic particles • Protons, neutrons, and electrons

21. Atomic Particles

22. Atomic Number Atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element. Identifies the atom.

23. Isotopes Elements occur in nature as mixtures of isotopes. Isotopes are atoms of the same element that differ in the number of neutrons

24. Isotopes…Again (must be on the test) Isotopes are atoms of the same element having different masses due to varying numbers of neutrons.

25. Mass Number Mass number is the number of protons and neutrons in the nucleus of an isotope. Mass # = p+ + n0 18 8 8 18 Arsenic 75 33 75 Phosphorus 16 15 31

26. Atomic Masses Atomic mass is the average of all the naturally isotopes of that element. On Periodic Table Carbon = 12.0125 amu

27. Practice Problems • P. 104 # 15-17

28. Writing Nuclear Symbols He 3 Mass # Atomic Symbol Atomic # 2 How many protons, electrons, and neutrons? 2 protons, 2 electrons, 1 neutron Mass # - Atomic # = # Neutrons

29. Writing Isotopes Using Hyphen Notation Uranium-235, Helium-3, or Carbon-14 How many proton, electrons, neutrons? Name of atom Mass # 92 protons, 143 neutrons, 92 electrons

30. Isotope problems Convert these hyphen notation to nuclear symbols. Uranium-235, Helium-3, or Carbon-14 235 U He 14 C 3 92 6 2