Transcript Discovering Atomic Structure - U

Discovering Atomic
Structure
Chapter 3-1
Early Models of the Atom
• Cut a piece of Al foil in ½ again and
again…….
• 450BC Democritus , Atomos
• The ATOM: Smallest piece of an
element that retains identity of that
element.
• But what holds them together?
• Lavoisier – Conservation of matter.
• Proust – Law of constant composition
(compounds always have the same
proportions of the elements
comprising it).
–Ex. Water 88.9% O and 11.1% H.
• Dalton’s Atomic Theory:
–Extremely small particles (atoms).
–Atoms of a given element are
identical but different from all other
elements.
–Atoms never created nor destroyed.
–compounds always have the same
proportions of the elements
comprising it
• Recent advances (the scanning-tunneling
microscope) have almost allowed us to
“see” atoms. Daltons postulates have been
proven as fact.
Scanning tunneling microscope
• John Dalton had believed atoms to be
hard spheres like marbles.
• 1839 – Faraday suggested electrical
phenomena related to atomic structure.
• Elektron (amber), static (stationary)
electricity.
• Ben Franklin. 2 types of charge, + or - .
–Opposites attract, like charges repel.
• So, a + and a + do what?
• REPEL
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So, a - and a - do what?
REPEL
So, a + and a - do what?
ATTRACT
But what about a – and a + ??????
ATTRACT
Cathode Ray Tube – gave us
an understanding of electrons
Cathode Ray Tubes (contd.)
• The tubes would glow when the end
screen was coated with some
substances
• The ‘cathode rays’ could spin a paddle
wheel so they must have had mass
(particles)
• A magnet deflects the particles just like
negatively charged objects
• Thompson was able to use magnets to
deflect the cathode rays. He called the
particles electrons.
• He could calculate the ratio of
Electron Charge
Electron Mass
• Millikan measured the Electron charge.
• Solving, the electron mass was found to
be 1/1800 mass of a hydrogen atom.
• At this time most believed in the “plum
pudding” model of the atom
• RADIOACTIVITY: (led to understanding of
nucleus)
• Bequerel had some mysteriously exposed
photographic plates. Uranium- radioactive
• Curies isolated Radium and Polonium
• Rutherford – radioactive substances show
changes in their properties with time.
• His experiment showed + alpha particles,
- beta particles and uncharged Gamma
radiation
• Thompson showed that most atoms are
neither net (+) or (–)
• But (–) electrons must imply a positive
part of the atom , the nucleus
• The further gold foil experiment proved the
true nature of atoms.
Rutherford gold foil experiment
• http://www.mhhe.com/physsci/chemistry/essenti
alchemistry/flash/ruther14.swf
• A very small, dense core (nucleus)
containing + charged “protons” and
uncharged neutrons
• A lot of space in which electrons traveled
– the electron cloud, surrounds the
nucleus.