Atomos: Not to Be Cut

1
Atomos Not to Be Cut

• The History of Atomic Theory

2
Who are these men and what are they doing in our
classroom?
3
Democritus
Aristotle
4
Democritus
400 BC

• began the search more than 2400 years ago.
• proposed that matter could not be divided into
  smaller and smaller pieces forever
• named the smallest piece of matter atomos,
  meaning not to be cut.

5

• This theory was ignored and forgotten for
  more than 2000 years!

6
Why?

• Aristotle and Plato, had a more respected, but
  wrong, theory.

Aristotle and Plato favored the earth, fire, air
and water approach to the nature of matter. Their
ideas held sway because of their eminence as
philosophers. The atomos idea was buried for
approximately 2000 years.
7
Ancient Philosophy

• Who Aristotle, Democritus
• When More than 2000 years ago
• Where Greece
• What Aristotle believed in 4 elements Earth,
  Air, Fire, and Water. Democritus believed that
  matter was made of small particles he named
  atoms.
• Why Aristotle and Democritus used observation
  and inference to explain the existence of
  everything.

8
Alchemists

• Who European Scientists
• When 800 900 years ago
• Where Europe
• What Their work developed into what is now
  modern chemistry.
• Why Trying to change ordinary materials into
  gold.

9
Alchemic Symbols
10
John Dalton
11
Daltons Theory

• All elements are composed of atoms.
• Atoms are indivisible and indestructible
  particles.
• Atoms of the same element are exactly alike.
• Atoms of different elements are different.
• Compounds are formed by the joining of atoms of
  two or more elements.

12
Particle Theory

• Who John Dalton
• When 1808
• Where England
• What Described atoms as tiny particles that
  could not be divided. Thought each element was
  made of its own kind of atom.
• Why Building on the ideas of Democritus in
  ancient Greece.

13
.

• This theory became one of the foundations of
  modern chemistry.

14
J. J. Thompson
15
Thomson Model

• Plum Pudding model.
• Atoms were made from a positively charged
  substance with negatively charged electrons
  scattered about, like raisins in a pudding.

16
Thomson Model

• Thomson studied the passage of an electric
  current through a gas.
• As the current passed through the gas, it gave
  off rays of negatively charged particles.

17
Section 1 Development of the Atomic Theory
Chapter 11
18
Thomson Model
Where did they come from?

• This surprised Thomson, because the atoms of the
  gas were uncharged. Where had the negative
  charges come from?

19
Discovery of Electrons

• Who J. J. Thompson
• When 1897
• Where England
• What Thompson discovered that electrons were
  smaller particles of an atom and were negatively
  charged.
• Why Thompson knew atoms were neutrally charged,
  but couldnt find the positive particle.

20
Ernest Rutherford
21

• He fired a stream of tiny positively charged
  particles at a thin sheet of gold foil (2000
  atoms thick)

22
Section 1 Development of the Atomic Theory
Chapter 11
23
What did this mean?

• all of an atoms positively charged particles
  were contained in the nucleus
• The negatively charged particles were scattered
  outside the nucleus around the atoms edge.

24
Atomic Structure I

• Who Ernest Rutherford
• When 1911
• Where England
• What Conducted an experiment to isolate the
  positive particles in an atom. Decided that the
  atoms were mostly empty space, but had a dense
  central core.
• Why He knew that atoms had positive and negative
  particles, but could not decide how they were
  arranged.

25
Niels Bohr
26
Bohr Model

• placed each electron in a specific energy level.
• electrons move in definite orbits around the
  nucleus
• These paths are located at certain distances from
  the nucleus.

27
Atomic Structure II

• Who Niels Bohr
• When 1913
• Where England
• What Proposed that electrons traveled in fixed
  paths around the nucleus. Scientists still use
  the Bohr model to show the number of electrons in
  each orbit around the nucleus.
• Why Bohr was trying to show why the negative
  electrons were not drawn into the nucleus of the
  atom.

28
ATOMIC STRUCTURE
S.MORRIS 2006
29
The Wave Model

• based on the principles of wave mechanics.
• electrons do not move about an atom in a definite
  path, like the planets around the sun.

30
The Wave Model

• it is impossible to determine the exact location
  of an electron. The probable location of an
  electron is based on how much energy the electron
  has.
• An atom has a small positively charged nucleus
  surrounded by a large region in which there are
  enough electrons to make an atom neutral.

31
Electron Cloud

• A space in which electrons are likely to be
  found.
• Electrons whirl about the nucleus billions of
  times in one second
• They are not moving around in random patterns.
• Location of electrons depends upon how much
  energy the electron has.

32
Electron Cloud

• Depending on their energy they are locked into a
  certain area in the cloud.
• Electrons with the lowest energy are found in the
  energy level closest to the nucleus
• Electrons with the highest energy are found in
  the outermost energy levels, farther from the
  nucleus.

33
Electron Cloud Model

• Electrons travel around the nucleus in random
  orbits.
• Scientists cannot predict where they will be at
  any given moment.
• Electrons travel so fast, they appear to form a
  cloud around the nucleus.

34
(No Transcript)
35
Indivisible Electron Nucleus Orbit Electron Cloud
Greek X
Dalton X
Thomson X
Rutherford X X
Bohr X X X
Wave X X X