Democritus (460-370 B.C.)

1. Democritus (460-370 B.C.) Democritus was one of a few Greek philosophers who believed that all matter in the world was made of of indivisible parts which he called “atomos,” which means “indivisible.” Although this theory was to be discovered to later be the truth, Democritus’ ideas faded until the seventeenth century in Europe. Mullis

2. John Dalton (1766-1844) • Dalton is known for his atomic theory • Theory states that atoms are the smallest chemical building block of nature • His four postulates: • Each element is composed of extremely small particles called atoms • All atoms of a given element are identical; the atoms of different elements are different and have different properties (including different masses) • Atoms of an element are not changed into different types of atoms by chemical reactions; atoms are neither created nor destroyed in chemical reactions • Compounds are formed when atoms of more than one element combine; a given compound always has the same ratio and kind of atoms • ATOMS ARE SMALL AND CANNOT BE DIVIDED (later changed!) http://en.wikipedia.org/wiki/John_Dalton#Atomic_theory Mullis

3. Discovery of Atomic Structure • By 1850 scientists knew that atoms were composed of charged particles. • Electrostatic attraction: • Like charges repel • Opposites attract Mullis

4. J.J. Thomson • Experimented with cathode rays and found that the properties didn’t change • Distinguished charges within atoms, positive and negative charges • Plum-pudding model • Found electron charge ratio Mullis

5. Cathode Rays and Electrons • C.R. 1st discovered in mid-1880s from studies of electrical discharge thru partially evacuated tubes (CRTs) • Cathode rays = radiation produced when high voltage is applied across the tube. • The voltage causes negative particles to move from the negative electrode (cathode) to the positive electrode (anode). • The path of electrons can be altered by the presence of a magnetic field. Mullis

6. Consider cathode rays leaving the positive electrode through a small hole…. • If they interact with a magnetic field perpendicular to an applied electric field, then the cathode rays can be deflected by different amounts. • Amount of deflection depends on applied magnetic and electric fields. • Deflection also depends on the charge-to-mass ratio of an electron. • Thomson determined the charge-to-mass ratio of an electron in 1897. • Charge-to-mass ratio = 1.76 x 108 C/g • C: Coulomb, SI unit of electric charge Mullis

7. Robert Millikan • Millikan built on J.J. Thomson’s work on electrons by measuring their charge with his famous oil-drop experiment. • Using Thomson’s ratio, Millikan calculated electron’s mass of 9.10 x 10-28 g which proved how much smaller electrons are than nucleus particles. Mullis

8. Millikan Oil-Drop Experiment • Sprayed oil drops over the hole in a positively charged plate and measured the electrostatic force of attraction. • Found the charge on the electron to determine its mass • Concluded the charge on the electron must be 1.60 x 10-19 C • Mass of electron = 1.60 x 10-19 C = 9.10 x 10 -28 g 1.76 x 108 C/g Mullis

9. Radioactivity(Spontaneous emission of radiation) Mullis

10. Ernest Rutherford -Conducted an experiment using alpha particles to bombard gold foil to conclude: 1. Nucleus of an atom contains positive particles that we now call protons. • The nucleus is a small dense area in the atom. -Studied three types of radioactive emissions: alpha, beta & gamma -Concluded that alpha particles were He nuclei Mullis

11. A Positively Charged Nucleus • Rutherford shot alpha particles though a thin piece of gold foil. • Some of these particles were deflected instead of passing straight through • Recall “like repels like.” • When a + alpha particle encountered a nucleus of a gold atom, it was deflected by the dense positively charged nucleus. Mullis

12. neutron James Chadwick • James Chadwick suspected there were neutral particles when he saw in experiments like Rutherford’s that some of the particles were deflected backwards, meaning that they had no charge. Chadwick had discovered the neutron! Mullis

13. Niels Bohr • Bohr proposed that an electron circles the nucleus in allowed orbits at specific energy levels. orbital Mullis Chemistry Holt Ch.4

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15. Scientist Contributions • Thomson: • “Discovered” electron (1897) • Cathode ray experiments • “Plum pudding” atomic model • Millikan: • Mass of electron • Oil-drop experiment (1909) • Rutherford: • Positively charged nucleus (1911) • Gold foil experiments • Discovered proton (1919) • Chadwick: Discovered neutron (1932) Mullis

16. Small Numbers • Electronic Charge: 1.609 x 10-19 C • Charge on an electron: -1.609 x 10-19 C • Charge on a proton: +1.609 x 10-19 C • Atomic Mass Unit (amu): 1.66054 x 10-24 g • Proton mass: 1.0073 amu • Neutron mass: 1.0087 amu • Electron mass: 5.486 x 10-4 amu • Unit of length used to note atomic dimensions: 1 Angstrom(Å) = 1x10-10 m Mullis

17. Atomic Number • Number of protons or electrons in an element • Identifies the element • Atomic Mass • Nucleus contains most of the mass of an atom. • Protons and neutrons are each ~ 1.67 x 10-24 g. • Electrons are each ~ 9.11 x 10-28 g. • Use atomic mass unit (amu) instead of gram. • The mass of one proton is ~ 1 amu. • Mass Number • The sum of the number of protons and number of neutrons in the nucleus • Is approximately equal to the average atomic mass shown on periodic table. • Number of neutrons = mass number – atomic number Mullis

18. Isotopes • Atoms of the same element with different numbers of neutrons • Have the same number of protons • Example: Carbon-12 and Carbon-14 • Radioactive Isotopes • Unstable in nature • Can be used to date fossils and rocks • The time it takes for half of the radioactive atoms in a piece of the fossil to change to another element is its half-life. Mullis

19. Isotopes AX Z • Isotopes have the same Z, but different A. • Isotopes have different numbers of neutrons. • An atom of a specific isotope is called a nuclide. • Nuclidesof hydrogen include: • 1H = hydrogen (protium) • 2H = deuterium (heavy hydrogen) • 3H = tritium (3H is radioactive.) Mullis

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