1. Chapter 16: The Atom

2. The Atomic Model

3. Model of the Atom
The atomic model is an idea of what an atom looks like.
It is not a model like a model airplane.
Scientific models are simply working representations.

4. Model of the Atom There are two different types of models. Conceptual
Mathematical

5. Model of the Atom
Scientific models of any kind are just tools to help us visualize real-life phenomena.

6. History of the Atomic Model
Leucippus

7. Leucippus
Leucippus was a Greek who believed that there is a smallest particle.
This belief implies that the atom is indivisible.

8. History of the Atomic Model
Leucippus
Democritus

9. Democritus Democritus was a student of Leucippus.
He called the smallest particle the atom. (Atomos means “indivisible.”)
He believed that atoms make up all matter.

11. Democritus Democritus was right!
There is a basic unit of matter called the atom.
However, the atom is not indivisible, and it has not always existed.

12. Democritus
Democritus made no observations, so he cannot be called a scientist.
He reasoned, not experimented, to create his model.

13. What did Democritus do? He discovered the atom.
He theorized that atoms exist.
He discovered the electron.
He discovered the proton.

14. Democritus was right when he said
matter is composed of atoms.
atoms are indestructible.
atoms are large.
matter is composed of protons and electrons.

15. The Atom Rediscovered
As more information was discovered about the atom, scientists made changes to the atomic model.
Model change represents a revolution in the way scientists are thinking.

16. History of the Atomic Model
Leucippus
Democritus
Dalton

17. Dalton
Dalton created the first experimental model by weighing substances and applying the Law of Definite Proportions.

18. Dalton
The Law of Definite Proportions, discovered by Joseph Proust, states that every compound has a definite composition by mass.
For example, water’s composition by mass is a 1:8 ratio.

19. Dalton
Dalton used Proust’s Law and further experiments to come up with his model called the core-envelope model.

20. heat envelope
core

21. Dalton’s Model Atoms are neither created nor destroyed.
Atoms cannot be subdivided.

22. Dalton’s Model Elements are made only of atoms.
All atoms of an element are the same.
Atoms of elements differ in their masses.

23. History of the Atomic Model
Leucippus
Democritus
Dalton
Thomson

24. Thomson
Thomson found that the atom is not solid—it has negative particles in it.
He discovered the electron (e–).

25. Thomson’s Model
The electrons are in a positive (+) gel-like substance.
The electrons and the positive substance charges cancel out.
Electrons do not vary in mass or charge.
Electrons can be removed from the atom.

26. Thomson’s Model
Thomson’s model is known as the plum-pudding model.

27. atom
positive
“pudding”
negative
electrons
“plums”

28. Thomson’s Model
Thomson’s model can also be called the chocolate chip cookie model.

29. Thomson’s model looked like
peanut brittle.
roast beef.
plum pudding.
a hot dog.

30. Thomson’s cathode rays were
protons.
neutrons.
quarks.
electrons.

31. History of the Atomic Model
Leucippus
Democritus
Dalton
Thomson
Rutherford

32. Rutherford Rutherford shot alpha particles at thin gold foil.
Alpha particles are relatively heavy and positively charged.
Some of the alpha particles bounced straight back, revealing that atoms have a nucleus made of positive protons (p+).

33. glass plate
microscope
gold foil
vacuum
chamber
zinc
sulfide
screen
scattered
alpha particles
alpha
particle
beam
rotating
housing

34. Rutherford’s Model
Rutherford’s model is called the nuclear model.

35. Rutherford’s Model
Protons form a small, central nucleus only 1/100,000 the size of the atom.
Protons are many times larger than electrons.
Electrons are whirling somewhere around the nucleus.
Most of the atom is empty space.

36. electrons
occupy space
around
nucleus
tiny dense
positive
nucleus

37. Rutherford’s experiment involved
a cathode ray.
alpha particles.
neutron emission.
atomic absorption diffraction.

38. Rutherford discovered
protons.
neutrons.
quarks.
electrons.

39. History of the Atomic Model
Leucippus
Democritus
Dalton
Thomson
Rutherford
Chadwick

40. Chadwick Chadwick discovered the neutron (n°).
The neutron is as big as the proton but it has no charge.
The neutron is also in the nucleus.

41. History of the Atomic Model
Leucippus
Democritus
Dalton
Thomson
Rutherford
Chadwick
Bohr

42. Bohr
Bohr knew that atoms give off colors when heated (like fireworks).
Looking through a prism, he could see different colors being emitted.

43. Flame Tests

44. Bohr
Bohr thought that the electrons jumped further from the nucleus when heated and then fell back down, giving off energy as they fell.
The electrons occupy specific levels at their normal energies.

45. Bohr’s Model
Each atom has a different arrangement of electrons, so it gives off different colors of light.
Electrons can jump from level to level.

46. Bohr’s Model
Bohr’s model looked like the solar system, so it was called the planetary model.

47. Kr

48. History of the Atomic Model
Leucippus
Democritus
Dalton
Thomson
Rutherford
Chadwick
Bohr
Quantum
model

49. Quantum Model
The quantum model replaces Bohr’s exact orbits with regions (like whirling fan blades) where electrons are most likely to be found.
These regions are called orbitals.