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 ( amu, abundance 78.99%), 25Mg ( amu abundance 10.00%), and 26Mg ( 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 = amu. The isotopic mass of Boron-10 = amu, and the isotopic mass of Boron-11 = 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