1. History and Structure of the Atom
From Democritus to...

2. History of Atomic Theory2
Democritus (from about 440 BC)
coined the term atom which means uncuttable
He felt that if you kept cutting matter smaller and smaller eventually you will no longer be able to cut any further.

3. John Dalton (1766-1844) Felt that an atom was indivisible3
Felt that an atom was indivisible
spherical in shape
Model: Sphere
Analogy: Billiard ball

4. Dalton’s Postulates Atoms are tiny, indivisible particles.
Atoms of the same element are identical.
Atoms of different elements can chemically combine.
Chemical reactions occur when atoms are separated, joined or rearranged.
Atoms of one element can never be changed to atoms of another element.

5. Size of an atom Pure copper coin the size of a penny = 2.4X1022 atoms
Earth’s population is about 6 X 109 people

6. JJ Thomson (1856-1940) Discovered electrons
Felt that an atom was negatively charged particles floating in a positive soup
Model: Charges floating around
Analogy: Raisin bun or Plum Pudding

7. Ernest Rutherford ( ) 7
Discovered the nucleus, the proton and first split an atom
Model: An atom was a small positively charged nucleus surrounded by electrons orbiting around it.
Analogy: A beehive

8. Rutherfords Famous Experiment8
Called the Gold Foil Experiment
Alpha particles were fired at thin gold foil
A detector encircled the foil and lit up when hit with alpha particles.
If the plum pudding model were true it was expected most particles would go straight through the foil with only slight deflection.
However, in the experiment, some particles were deflected back at a sharp angle proving the existence of a small, dense, and positively charged nucleus.

9. Plum pudding expected results. Particles go through.
Rutherfords results
Plum pudding expected results. Particles go through.
Text
Actual results. Some are deflected back due to a nucleus

10. Another view of the experiment10

11. Niels Bohr 11
Worked out details of atomic structure. Notably orbital layers. Solved problems related to Rutherfords model.
Model: Electrons orbit in rings at different distances from the nucleus.
Analogy: Planets orbiting the sun

12. Erwin Schroedinger (1887-1961)12
Major discoveries in quantum mechanics. Nature of electrons in atoms.
Model: Electrons exist in a probability distribution around the atom. Kind of like a cloud.
Analogy: A spinning fan blade.

13. Quick Quiz: Historical Atom13
1. What did Dalton say an atom was like?
2. What did JJ Thomson say an atom was like?
3. What did Rutherford say an atom was like?
4. What are the 3 parts of an atom?
1. A billiard ball, or a pool table ball
2. Raisin bun
3. A beehive
4. Proton, Neutron, Electron

14. Parts of the Atom 14
Electrons
The Nucleus

15. Parts of the Atom Electrons Mass=0 Charge=-1 Protons Mass=1 Charge=+115
Electrons
Mass=0
Charge=-1
Protons
Mass=1
Charge=+1
Neutrons
Mass=1
Charge=0
The Nucleus

16. Atomic Structure
An atom is considered electrically neutral. Electrically neutral means the number of protons (+) = the number of electrons (-)
4 red protons = 4 blue electrons

17. Atoms on the Periodic Table17
Atomic # = Number of protons 3 Be
7.0
neutrons
+ protons
Atomic mass

18. You must know how to find:
Nuclear Symbol
You must know how to find:
# of protons = atomic number
mass # = # of n0 + # of p+ (atomic #)
What’s in the nucleus of the atom
# of electrons = # of protons (in a neutral atom)
Boron 5 B
10.811
atomic number
(Not the same as the mass #)
atomic mass

19. 16. What is the mass number? The mass of the entire atom!
The sum of the protons and neutrons in an atom.
Electrons are not included (too small)

20. 17. What is the atomic mass?
The weighted average of the masses of all naturally occurring isotopes.

21. How to write a Nuclear Symbol
Charge if ion
Element Symbol B
Mass Number
= p+ + n0 -3 11 5
Atomic Number
= p+

22. To Determine Other Numbers22
To find
Do this
Proton #
same as atomic number
Electron #
same as atomic number for neutral atoms
Neutron #
=atomic mass-atomic number
Mass
=Prot # + Neut#

23. Cl Mg Nuclear Symbol Examples 35 17 27 +2 12 17 35 17 18 17 12 27 12
Number of Protons
Number of Neutrons
Number of Electrons
Atomic Number
Mass Number 17 35 17 18 17 Mg 27 +2 12
Number of Protons
Number of Neutrons
Number of Electrons
Atomic Number
Mass Number 12 27 12 15 10

24. Ca O Nuclear Symbol Examples 40 +2 20 17 -2 8 20 40 20 20 18 8 17 8 9
Number of Protons
Number of Neutrons
Number of Electrons
Atomic Number
Mass Number 20 40 20 20 18 O 17 -2 8
Number of Protons
Number of Neutrons
Number of Electrons
Atomic Number
Mass Number 8 17 8 9 10

25. Try This 25
Atomic #
Element Symbol
Atomic Mass Be Na Fe I

26. Fill in the blanks 26
Element
Atomic #
e- #
Mass #
Neut # 12 P 15 16 8

27. Foldable

28. Ions - an atom that has lost or gained an electron(s)
The charge on an ion indicates an imbalance between protons and electrons.
If the atom GAINS electrons it will have a negative charge. (more e- than p+)
If the atom LOSES electrons it will have a positive charge. (more p+ than e-)
Protons =
Electrons =
Protons = B 11 B
Electrons = 11 +2 5 5

29. Isotopes
Isotopes are atoms of the same element that have the same # p+, but different mass numbers. Isotopes have different masses because they have a different number of neutrons.
Boron 5 B
10.811
atomic number
(average) atomic mass

30. 19. How are isotopes written?
The element name with its mass number after it:
Carbon-12
Carbon-13
Carbon-14

31. Learning Check
Naturally occurring carbon consists of three isotopes, C-12, C-13, and C-14. State the number of protons, neutrons, and electrons in each of these carbon atoms.
12C C C
# p _______ _______ _______
#n _______ _______ _______
#e _______ _______ _______ #

32. The average atomic mass - weighted average of all the naturally occurring isotopes of an element.
Example:
A sample of cesium is 75% 133Cs, 20% 132Cs and 5% 134Cs. What is the average atomic mass?
Answer:
.75 x 133 = 99.75
.20 x 132 = 26.4
.05 x 134 = 6.7
= average atomic mass