History behind the atomic structure and key concepts of formation of atoms.

Contributed by:

kevin Mon, Jan 24, 2022 06:03 AM UTC

Atomic structure refers to the structure of an atom comprising a nucleus (center) in which the protons (positively charged) and neutrons (neutral) are present. The negatively charged particles called electrons revolve around the center of the nucleus.

1. Atomic Structure

2. Democritus 460 BC
► GreekPhilosopher
► 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

3. 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

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

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

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

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

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

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

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

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

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

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

20. Thomson’s Experiment
Voltage source
+
-
-
 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
He is credited with the discovery of
protons in an atom.
Canal Rays discovery

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

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

26.

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

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

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
► Protonshave a positive (+) charge and
electrons have a negative (-) charge
► Ina 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

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

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
► NuclearSymbol
notation (no periodic
table)
108
A
 Protons =
 Electrons =
47
 Neutrons =


Atomic Number =
Atomic Mass = g

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

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

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