Unit 2 History of the Atom.notebook
Warm Up 1: CH 4 RG or OL ON DESK February 05, 2012
I/E – Review Unit 1 Test
What is the difference between an element, cmpd, atom, and subatomic particle?
Element
Cmpd
Atom Subatom Part
Jan 16­9:50 AM
Historic Models of the Atom
Jan 16­9:50 AM
The Beginning of the Atom
• Democritus’s Atomic Philosophy 400BC
~ Believed atoms were indivisible and indestructible
Dalton took it and ran with it 2000 yrs later!
Jan 16­9:50 AM
Jan 16­9:50 AM
Historical Models of the Atom
??
E?
RU
T
Dalton: circa 1800:
• Atoms are indivisible
• Atoms of 1 element are different from atoms of another element
• Atoms combine in simple whole number ratios (Law of Definite Proportions) • During reactions atoms are rearranged but unchanged
Jan 16­9:50 AM
http://www.youtube.com/watch?v=O9Goyscbazk
Cathode Ray Tube (CRT)
At low pressures, current flows through a gas.
Experimentation shows:
• Glow on the glass opposite the cathode
• Different gases cause different color glows
• Paddle wheel in the middle spins
• electron—postulated to explain electricity!! Conclusions:
Cathode rays are composed of particles.
CRs are deflected by external electric and magnetic fields.
Jan 16­9:50 AM
1
Unit 2 History of the Atom.notebook
February 05, 2012
• Deflected away from the negative plate
• Therefore: particles are negative!! JJ Thomson (1897﴿ • Concluded that CR were beams of identical, negatively charged particles .... called them electrons!! • Studied deflection of cathode rays
• Heavier (subatomic) particles were deflected less
• More charge = more deflection (why… b/c repulsion!﴿
• e‐ has mass and a charge; its mass‐to‐charge ratio was much smaller than that of the hydrogen ion
• 2 particles with the same mass‐to‐charge ratio move in the same path in a vacuum when subjected to the same electric and magnetic fields.
Jan 16­9:50 AM
Jan 16­9:50 AM
Robert Milikan (1909)
Preformed the oil drop experiment
• Drops fall due to gravity
• X‐ray put a negative charge on drops
• He measure charge on the drops
• Measured the amt of charge for each e‐ then using Thomson’s ratio calculated the mass of an electron... 9.10938291﴾40﴿×10−31 kg
Jan 16­9:50 AM
Ernest Rutherford (1912)
performed the gold foil experiment
Eugene Goldstein
Modified a CRT and found
positive charge in the atom too
~2000X heavier than the e-
Jan 16­9:50 AM
Rutherford’s model:
The positive center was called the nucleus
(w/ protons and neutral particles) Most of the atom is empty space ; e‐ are outside of the nucleus
~Predicted that alpha particles would pass through b/c atom is mostly empty space
~99% did – 1 % were deflected
~concluded that there was a concentrated positive center (w/all mass!) in the atom... called proton! Jan 16­9:50 AM
Jan 16­9:50 AM
2
Unit 2 History of the Atom.notebook
James Chadwick (1932)
February 05, 2012
The Elements - http://www.youtube.com/watch?v=zGM-wSKFBpo
http://www.youtube.com/watch?v=zGM­wSKFBpo
discovered the neutron suspected purpose is to hold the protons in the nucleus together Jan 16­9:50 AM
Jan 16­9:50 AM
The Atomic Timeline
• Work with a partner of your choice
• Start on paper – plan it out and leave room for the Modern Models and scientists
Jan 16­9:50 AM
3