history of atomic theory. essential question how did the atomic theory evolve from dalton's...
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History of Atomic Theory
Essential Question
• How did the atomic theory evolve from Dalton's Postulates to the current modern atomic theory?
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.
Democritus• Matter is composed of empty space through which
atoms move.• Atoms are solid, homogeneous, indestructible,
and indivisible.• Different kinds of atoms have different sizes and
shapes.• The different properties of matter are due to the
size, shape, and movement of atoms.• Apparent changes in matter result from changes
in the groupings of atoms and not from changes in the atoms themselves.
Aristotle
Democritus
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.
Alchemic Symbols
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.
John Dalton• Matter composed of small particles (atoms)• Atoms of an element have the same size, mass, and
chemical properties.• Atoms cannot be created, divided into smaller particles,
or destroyed.• Atoms of different elements combine in whole number
ratios to form compounds (H2O, CO2, MgO, etc.).
• In a chemical reaction, atoms are separated, combined or rearranged.
John Dalton
Discovery of Electrons• Who: J. J. Thomson
• When: 1897
• Where: England
• What: Thomson discovered that electrons were smaller particles of an atom and were negatively charged.
• Why: Thomson knew atoms were neutrally charged, but couldn’t find the positive particle.
Cathode Ray Tube- Cathode rays are a stream
of charged particles
- Particles carry a negative charge (electrons)
- Thomson concluded that electrons were smaller than a hydrogen atom (lightest known atom)
- Conclusion: Electrons are smaller than atoms
- Dalton was wrong: Atoms were divisible into smaller subatomic particles
The Atom, circa 1900:
• “Plum pudding” model, put forward by Thomson.
• Positive sphere of matter with negative electrons imbedded in it.
Plum
Pudding
J. J. Thomson
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.
Discovery of the Nucleus:Gold Foil Experiment
Ernest Rutherford shot particles at a thin sheet of gold foil and observed the pattern of scatter of the particles.
(Zinc sulfide)
The Nuclear Atom
Since some particles were deflected at large angles, Thomson’s model could not be correct.
THE NUCLEAR MODELAccording to Rutherford,
most of the atom consists of electrons moving rapidly through empty space and a very dense, tiny, positively charged nucleus.
Ernest Rutherford
Other Subatomic Particles• By 1920 , Rutherford had refined his
concept of the nucleus: He concluded that the very dense nucleus contained positively particles called protons.
• James Chadwick (1932), a coworker, showed that the nucleus also contained a neutral particle. This was the neutron – a particle with nearly equal mass as a proton.
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 sucked into the nucleus of the atom.
Bohr Model
THE BOHR MODEL
Niels Bohr
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.
Electron Cloud Model
Properties of Subatomic Particles
Particle Symbol Location ChargeRelative
massActual mass
(g)
Electron e- Around nucleus
1– 1
18409.11×10-28
Proton p+ Nucleus 1+ 1 1.673×10-24
Neutron n0 nucleus 0 1 1.675×10-24
Atomic Structure TimelineRequirements:• Must include a title.• Must include a spot for each of the 8 items.• Must include at least the Who, When, and
What.• Must have a picture (include color) for each
item.Options:1. Comic book format2. Standard timeline
Atomic Structure and the Periodic Table
14
Si
28.086
Silicon
Atomic number = # protons
Symbol
NameAtomic Mass: Round to a whole #:
28 = Relative mass of nucleus
-14 = # protons
14 = # neutrons
Practice From the Periodic Table
Complete the following chart:
Name SymbolRelative
mass#
protons#
neutrons#
electrons
Lithium
Mg
26
16
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