Chemistry

1. Atomic Structure

2. Democritus 460 BC
►Greek Philosopher
►Suggested world was made of
two things – empty space and
“atomos”
 Atomos – Greek word for
uncuttable
►2 Main ideas
 Atoms are the smallest possible
particle of matter
 There are different types of
atoms for each material

3. 2
John Dalton’s Atomic Theory
1804
1. All matter is made of atoms.
2. Atoms of one element are all
the same.
3. Atoms cannot be broken down
into smaller parts
4. Compounds form by
combining atoms

4. Dalton’s Early Atomic Model
►“Billiard Ball” model
►he envisioned atoms as solid, hard spheres,
like billiard(pool) balls, so he used
wooden balls to model them

5. J.J. Thomson
1897
Discovered the electron
He was the first scientist to show
the atom was made of even smaller
things

6. JJ Thomson
►Used the Cathode ray tube to discover
electrons

7. Thomson’s Experiment
Voltage source
+
-
Vacuum tube
Metal Disks

8. Thomson’s Experiment
Voltage source
+
-

9. Thomson’s Experiment
Voltage source
+
-

10.  Passing an electric current makes a
beam appear to move from the
negative to the positive end
Thomson’s Experiment
Voltage source
+
-

11.  Passing an electric current makes a
beam appear to move from the
negative to the positive end
Thomson’s Experiment
Voltage source
+
-

12.  Passing an electric current makes a
beam appear to move from the
negative to the positive end
Thomson’s Experiment
Voltage source
+
-

13.  Passing an electric current makes a
beam appear to move from the
negative to the positive end
Thomson’s Experiment
Voltage source
+
-

14. Voltage source
Thomson’s Experiment
►By adding an electric field

15. Voltage source
Thomson’s Experiment
 By adding an electric field
+
-

16. Voltage source
Thomson’s Experiment
 By adding an electric field
+
-

17. Voltage source
Thomson’s Experiment
 By adding an electric field
+
-

18. Voltage source
Thomson’s Experiment
 By adding an electric field
+
-

19. Voltage source
Thomson’s Experiment
 By adding an electric field
+
-

20. Voltage source
Thomson’s Experiment
 Adding an electric field cause the beam to
move toward the positive plate.
 Thomson concluded the beam was made of
negative moving pieces.
+
- -

21. Thomson’s “Plum Pudding” Atom Model

22. Democritus, Dalton, Thomson Video

23. Eugen Goldstein 1850-1930
Using a cathode ray tube he discovered
canal rays which are beams of positively
charged particles.
He is credited with the discovery of
protons in an atom.
Canal Rays discovery

24. Millikan’s Oil Drop Experiment
• Determined the charge on an electron
• Used Thomson’s charge to mass ratio to
calculate the mass of an electron
Millikan’s Oil Drop
Experiment
Millikan’s Oil Drop
Experiment – another
video

25. Ernest Rutherford - 1913
• discovered the
nucleus of a
gold atom with
his “gold foil”
experiment

27. Using J.J Thomson’s Plum Pudding
atomic model, Rutherford predicted the
alpha particles would pass straight
though the gold foil. That’s not what
happened.

28. Gold Foil Experiment Results
most alpha particles go straight through
the gold foil
A few alpha particles are sharply deflected

29. Rutherford’s Conclusion
►The atom is mostly
empty space.
►There is a small,
dense center with a
positive charge.
►Rutherford
discovered the
nucleus in atoms

30. Rutherford’s Contribution to the
Atomic Theory
►The atom is
mostly empty
space.
►The nucleus is
a small, dense
core with a
positive charge.

31. Gold Foil video

32. Rutherford’s Atomic Model

33. Structure of the Atom
► Atom – smallest particle of an element that can
exist alone
 Two regions of an atom
►Nucleus
 Center of atom
 Protons and neutrons
►Electron “cloud”
 Area surrounding nucleus
containing electrons

34. Structure of the Atom
►Proton – Positive charge (+), 1 atomic
mass unit (amu); found in the nucleus
 amu -Approximate mass of a proton or a
neutron
►Neutron – Neutral charge (0), 1 amu;
found in the nucleus
►Electron – Negative charge (-), mass is
VERY small

35. Counting Atoms
►Atomic Number
 Number of protons in nucleus
 The number of protons determines identity
of the element!!
►Mass Number (Atomic Mass)
 Number of protons + neutrons
 Units are g/mol

36. Counting Atoms
►Isotopes
 Atoms of the same element with varying
number of neutrons
 Different isotopes have different mass numbers
because the number of neutrons is different

37. Composition of the atom video
Start at 3:25

38. Counting Atoms
 Nuclear Symbol Notation

39. Atoms
►Protons have a positive (+) charge and
electrons have a negative (-) charge
►In a neutral atom, the number of protons
equals the number of electrons, so the
overall charge is zero (0)
 Example/ Helium, with an atomic number of 2,
has 2 protons and 2 electrons when stable

40. Ions
►In a neutral atom
 Atomic number = # of protons = #of electrons
►Sometimes atoms will gain or lose electrons
and form IONS
►Because an electron has a negative charge:
 When an atom GAINS electrons it becomes
NEGATIVE
 When an atom LOSES electrons it becomes
POSITIVE

41. Ions
Cation = a positive ion
Anion = a negative ion

42. Let’s Practice
►Aluminum (Al) (no periodic table)
 Protons = 13
 Electrons =
 Neutrons = 14
 Atomic Number =
 Atomic Mass =

43. Let’s Practice w/ nuclear symbol
notation
►Nuclear Symbol notation
(no periodic table)
 Protons =
 Electrons =
 Neutrons =
 Atomic Number =
 Atomic Mass =
Ag
108
47

44. Let’s Practice w/ isotopes
►use the periodic table
 Protons =
 Electrons =
 Neutrons =
 Atomic Number =
 Atomic Mass =
Uranium-235

45. Let’s Practice with Ions
Use the periodic table
 Charge =
 Protons =
 Electrons =
 Neutrons =
 Atomic Number =
 Atomic Mass =
K
39 1+