1. Atomic Structure
The Idea of the Atom

2. Early Models
Greek philosophers – 450 B.C. what is the smallest particle?
Democritus - Particles are atomos
An atom is the smallest particle of an element that retains the chemical identity of that element.

3. Aristotle Wins Greek society - slave based
Beneath them to work with hands – no experiments
Settled disagreements by arguments
Aristotle more famous than Democritus Aristotle wins
Didn’t believe or look into atoms until…

4. Late 1700s Chemists believed elements:
couldn’t be broken down by ordinary means
Combine to form compounds
Advancements in balances  better experiments

5. Laws Discovered
Law of Conservation of mass: mass is neither created nor destroyed during chemical or physical changes
Then discovered no matter where or how a compound is made it is made of fixed proportions of elements  Law of Definite Proportions
Ex: Sodium Chloride, table salt, always has 39.34% Na and 60.66% Cl by mass

6. Laws Discovered
Elements combine to form more than one compound Ex: carbon monoxide, CO, and carbon dioxide CO2
Law of Multiple Proportions

7. John Dalton 1808 –DALTON’S ATOMIC THEORY
All matter is composed of small particles called atoms.
*All atoms of a given element are identical, but they differ from those of any other element.

8. Dalton’s Atomic Theory
*Atoms cannot be subdivided, created, or destroyed
Atoms of different elements combine in whole-# ratios to form compounds
In a chemical reaction, atoms are combined, separated, or rearranged.

9. 1800s – The atom IS divisible
Atom has two regions:
Nucleus: center of atom
Protons: (+) charged
Neutrons: no charge
Electron cloud: region around nucleus containing electrons, (-) charged
Subatomic particles: p+, no, and e-

10. - + J.J. Thomson’s Experiment Cathode - Ray Tube Voltage source
Moving electrical current (Cathode Ray) through glass tube with inert gas (Noble Gas)

11. J.J. Thomson’s Experiment
Voltage source + -
By adding magnets, found the moving (-) particles

12. Thomson’s Model Same properties no matter what element used
All atoms had ELECTRONS!!
Thomson’s Model Plum pudding: bunch of positive stuff, with the electrons able to be removed.

13. Ernest Rutherford - 1911 Atoms have e- = (-) charge
Atoms are neutral  need (+) to balance
Must be other particles to account for mass of atom…
Rutherford used alpha particles to test.
Alpha particle: (+) charge, (2 p+ and 2 no)
Experiment: beam of high speed alpha particles into thin gold foil

14. Lead block Uranium Gold Foil Florescent Screen
Mr. Green. Mr. Green’s Homepage. 10 Sept Aug. 2004

15. His prediction….
Mr. Green. Mr. Green’s Homepage. 10 Sept Aug. 2004

16. What he expected
Mr. Green. Mr. Green’s Homepage. 10 Sept Aug. 2004

17. What he happened
Mr. Green. Mr. Green’s Homepage. 10 Sept Aug. 2004

18. Results… Most particles passed through 1 in 8000 bounced back
Some particles completely reversed path!!!

19. Small dense, positive piece at center, nucleus
Plum-pudding is WRONG!
Atom is mostly empty
Small dense, positive piece at center, nucleus
Alpha particles are deflected by nucleus if they get close. +
Mr. Green. Mr. Green’s Homepage. 10 Sept Aug. 2004

20. +
Mr. Green. Mr. Green’s Homepage. 10 Sept Aug. 2004

21. Electron Cloud Model Current model of atom
Nucleus surrounded by negative cloud “electron cloud”

22. Size of Atom Atom = size of football stadium
Nucleus = smaller than a dime in center
Electron = each one smaller than Roosevelt’s eye on the dime
Atom is mostly empty space!

23. Composition of Atom
Proton has (+) charge equal in magnitude to (-) charge of electron
Atoms neutral  # p+ = # e-
p+ and no about same mass
e- = 1836 times smaller than p+
Nucleus has majority of mass
Nuclear Forces: short-range forces that hold nucleus together
p+-p+, p+-no, no-no

24. Counting Atoms
Atomic Number (Z): number of p+ of each atom of the element
Identifies Elements!!

25. Atomic Number What is the atomic number for… Hg
What is the element with …
20 p+
Neutral atoms  # p+ = # e-
In neutral atoms, atomic number = # e-

26. Isotopes
Def: atoms of same element with different # of no (different masses)
3 H isotopes:
Protium: 1 p %
Deuterium: 1 p+ 1 no %
Tritium: 1 p+ 2 no

27. Mass Number Use mass number to determine isotopes
Mass #: # of p+ and n0
Atom with 17 p+ and 20 no has mass # of…
Chlorine-37 – Hyphen Notation

28. Cl Writing Isotopes 37 17 Nuclear symbol: Mass number Elemental symbol
Atomic number

29. Finding # of Neutrons 27Al # of neutrons = mass # - atomic #
How many p+, no and e- are in…
Selenium – 79
27Al
Oxygen - 18 13

30. Isotopes
Write the complete chemical/nuclear symbol for the isotope with 21 protons, 24 neutrons, and 21 electrons.

31. Relative Mass of Atom Mass of Oxygen-16 = 2.656 x 10-23 g
Use relative atomic masses
Pick standard  other masses are expressed in relation to standard
Standard: Carbon –12 atom

32. Carbon - 12 Has mass of 12 atomic mass units (amu)
1 amu: is exactly 1/12 the mass of a carbon-12 atom
Carbon-12: how many p+ and no?
1 p+  1 amu ( amu)
1 no  1 amu ( amu)
1 e-  0 amu ( amu)

33. Carbon - 12 What element is 4 x mass of Carbon –12?
What element is 1/3 mass of Carbon –12?

34. Average Atomic Masses Most elements are mixture of isotopes
Mass of element is average mass of isotopes
We need the % of each isotope
Average Atomic Mass: weighted average of the atomic masses of the naturally occurring isotopes of an element.

35. Weighted Average Box with 100 marbles of 2 sizes:
25% have mass of 2.00g
75% have mass of 3.00g
What is the average mass of a marble?
2.75g (sig figs!!)

36. Average Atomic Mass
Or (atomic weight) found on periodic table (ROUND TO 2 DECIMAL PLACES!!)
Average Atomic Mass =
(% Iso. #1)(Mass Iso.#1) + (%Iso.#2)(Mass Iso.#2) + …
100

37. Average Atomic Mass Find the atomic mass of Li if…
7.5% is Lithium – 6 = amu
92.5% is Lithium – 7 = amu

38. Example – AAM = 16.00 amu Isotope Fractional Abundance
Oxygen – 16 (15.99 amu)
99.757%
Oxygen – 17 (17.00 amu)
.038%
Oxygen – x
.235%

39. The Mole

40. Quantities 1 dozen = 1 gross = 1 ream of paper =12
1 gross =
144
1 ream of paper =
500
In chemistry: 1 mole = x 1023

41. The Mole SI unit for amount of substance (mol)
Def: the number of particles in exactly 12 g of carbon – 12.
12 g of carbon –12 has x 1023 atoms
Avogadro’s #, after Amadeo Avogadro
The number is HUGE!!

42. Molar Mass Def: mass of 1 mole of a pure substance
1 mole Carbon –12 = 12 g
1 atom Carbon – 12 = 12 amu
Mass of 1 mole of He atoms?
4.00g/mol
Same as atomic mass from periodic table, different units

43. Molar Mass
So: 4.00g He, 6.94 g Li and g Hg all have x 1023 atoms = 1 mole!!!
Molar mass (g/mol)  conversion factor!!

44. Gram/Mole Conversions
Mass, in g, of 3.6 mol of C?
43g
How many moles are in 23.5 g S?
0.733 mol
p. 85 problems

45. Conversions with Avogadro’s #
Diatomic Molecules: elements that exist as 2-atom molecules in natural state
7 diatomic molecules
H2, O2, N2, F2, Cl2, Br2, I2
Remember!!! HONClBrIF

46. Conversions with Avogadro’s #
How many moles of oxygen are in 2.00x1022 molecules of oxygen?
How many atoms of sodium are in 3.80 mol of sodium?

47. Conversions with Avogadro’s #
How many molecules of hydrogen are in g of hydrogen?
Mass of 5.0x109 atoms of neon?
p.86 problems