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How do we know about atoms?

How do we know about atoms? . Atomic Theory. Democritus (460 BC- 370 BC). His mentor Leucippus Everything is composed of “atoms” Atoms are physically indivisible Between atoms is empty space Atoms are always in motion

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How do we know about atoms?

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  1. How do we know about atoms? Atomic Theory

  2. Democritus (460 BC- 370 BC) • His mentor Leucippus • Everything is composed of “atoms” • Atoms are physically indivisible • Between atoms is empty space • Atoms are always in motion • Infinite number of atoms and kinds of atoms which differ in shape and size

  3. John Dalton (1766-1844) • Postulated the following: • that all matter was made of some type of particle- called atoms which could not be broken up or destroyed • Atoms could combine with different atoms to form compounds • All atoms of the same element are the same size (mass) and have the same properties • Atoms can be rearranged during a chemical reaction to form new substances

  4. Discovery of Electron • JJ Thomson in 1897 • Cathode Ray Tubes- A cathode ray forms when high voltage is applied to a partially evacuated tube. The ray passes through a hole in the negative end of the tube and hits the coated end of the tube to produce a glow

  5. Plum Pudding Model • Know that there are electrons (negative) • Must be a positive part as well in order to be neutral

  6. Discovery of the Nucleus • Ernest Rutherford designed an experiment to test the Plum Pudding Model. • Expected all particles to go straight through the foil

  7. They didn’t all go straight through? • Why?

  8. New Model = Nucleus

  9. Nucleus • Contains the mass of the atom. • 20 years later, James Chadwick discovered the neutron because the entire mass of the nucleus could not be accounted for by just protons.

  10. Re-examining Dalton’s Atomic Theory • Which postulates had to be modified after JJ Thomson and Ernest Rutherford’s experiments?

  11. All atoms are the same size- isotopes • Atoms can’t be changed- nuclear reactions • All mater is made of atoms – a proton is still matter

  12. Atomic Symbol -

  13. Isotopes • All atoms of an element are identical in atomic number, but not in atomic mass. • Isotopes = different number of neutrons. • Electrons determine chemical properties of an element. • Can determine neutrons from atomic mass and atomic number

  14. Using the info on the PT • How many protons, neutrons and electrons does an atom of iron have?

  15. Atomic Masses on the PT • How come they have decimals? • Does a single neutron or proton weight exactly 1 amu?

  16. Atomic Mass Unit (amu) • The mass of an atom is measured relative to the mass of an atomic standard. • The standard = carbon-12 atom. • 1 carbon-12 atom has exactly 12 amu

  17. Atomic Mass • The mass represented on the periodic table is the average of all of the naturally occurring isotopes weighted accordingly to their abundances. • PROBLEMS: find the abundances of isotopes based on the atomic mass. OR predict the atomic mass based on the abundances of certain isotopes

  18. Finding Atomic Mass • Magnesium has 3 naturally occurring isotopes, 24Mg (23.9850 amu, abundance 78.99%), 25Mg (24.9858 amu abundance 10.00%), and 26Mg (25.9826 amu, abundance 11.01%). Calculate the atomic mass of magnesium.

  19. Sample Problem • Boron has 2 naturally occurring isotopes. Find the percent abundance of Boron-10 and Boron-11 given the atomic mass = 10.81 amu. The isotopic mass of Boron-10 = 10.0129 amu, and the isotopic mass of Boron-11 = 11.093 amu.

  20. Homework: • Pg 69: # 1 (a,b,c), 2, 9, 20 (a,b,c) 21, 22, 23 (a,b), 29, 32, 91 • Read about mass spectroscopy on page 48, and summarize what it is

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