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Unit 3 The History of the ATOMand Atomic Structures
+Democritus 460-370 B.C.Democritus 460-370 B.C.
Greek Philosopher
He called nature’s basic particle an atomos, based on the Greek word “indivisible.”
Had no evidence, so people didn’t take him seriously.
He considers education to be the noblest of pursuits, but cautioned that learning without sense leads to error. (THERE MUST BE A POINT!!!)
Aristotle said empty space did not exist & rejected Democritus; said matter was EARTH, WATER, AIR & FIRE
+John Dalton 1803Dalton’s Atomic Theory
1. All elements are composed of tiny indivisible particles called atoms.
ElementA2. Atoms of the same
element are identical. The atoms of any one element are different from those of any other elements.
+ Dalton’s Atomic Theory
3. Atoms of different elements can physically mix together or can chemically combine with one another in simple whole-number ratios to form compounds.
+ Dalton’s Atomic Theory
4. Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction.
+ J.J. Thomson 1897 (with Crookes & Millikan)
discovery of the electron (e-)
disproved Dalton’s theory; atoms DID have smaller parts after all
Used a cathode ray tube to prove that there were charged particles (stream of (-) particles) within the atom
+Thomson Model
Plum pudding model aka chocolate chip cookie model
Millikan found out that electrons were negatively charged and had a mass of 1/1840 of a hydrogen atom
+Video Links to Experiments
JJ Thomson’s Cathode Ray Tube
How does it work?
Rutherford’s Gold Foil Experiment What did he do?
+Ernest Rutherford 1911
Used alpha particles to shoot at gold foil. Got unexpected results. Something was deflecting particles back to the source.
Buried next to Sir Issac Newton and Lord Kelvin in Westminster Abbey.
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NucleusEmpty Space
Electrons
+Rutherford said, “Atoms have a nucleus!”
Dense & positively charged
Contains most of the mass of the atom
Contains the protons (positive charge; heavy)
Electrons are around the outside of the nucleus flying around the empty space
+Video Links to Experiments
JJ Thomson’s Cathode Ray Tube
How does it work?
Rutherford’s Gold Foil Experiment What did he do?
+James Chadwick
Discovered the neutron in 1932
Has a neutral charge
Found in the nucleus
About ½ of the mass of the nucleus
Wanted to know where the extra mass was coming from in an atom; shot particles @ beryllium
Eventually led to experiments to bombard atoms with neutrons –> uranium bombs!
+Niels Bohr - 1913
Created the Bohr model
Electrons travel in definite orbits/ energy levels around the nucleus
aka “Planetary Model” – orbit the nucleus like planets around the sun
Electrons have fixed amounts of energy “quanta”Low energy closer to nucleusHigh energy further from nucleus
+ Bohr Model
+Werner Heisenberg, 1927
It’s impossible to know both the location and the motion of an electron at the same time
Heisenberg Uncertainty Principle
+ Quantum Mechanical Model(aka wave mechanical model)
Erwin Schrodinger – 1926
Electrons DO NOT orbit the nucleus
Electrons DO have quanta of energy that determine placement in an atom
+Electron Cloud
Area where there is a high probability that the electron will reside there 90% of the time
+Schrodinger’s Cat
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Subatomic Particles
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Nucleus - protons - neutrons
Electron Cloud - electrons
++ oo- -
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p+ n0
e-Found in the nucleus
Mass = 1 (amu)
Relative charge = +1
Found in the nucleus
Same mass as a proton
Mass = 1 amu
Relative charge = 0
Found in the electron cloud around the nucleus
Mass 1836 times smaller than a proton
Therefore, mass doesn’t really count! (0.0005 amu)
Relative charge = -1
+ Atomic Number Atomic Number Z = p+
the number of protons in the nucleus
ALL atoms of the PROTONS SAME element have the IDENTIFY SAME atomic number and THE
ELEMENT! SAME protons
In a neutral atom, p+ = e-
Magnesium = Z = 12 12 protons
18.99
+Mass Number (nucleus!)
Mass number = p+ + n0
(electrons don’t have much mass!!!)
Z = 82 LEAD, Pb Mass 207
Protons? Electrons? Neutrons? 82 82 82 125
#n0 = MN - # p+
= 207 – 82= 125 neutrons
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An element has an atomic number of 6 and a mass number of 14. What is the element? How many neutrons?
Z = 6 CARBON
MN = 14 #n = MN – #p+
= 14 – 6 = 8 neutrons
+ 17
2
18.99
Average atomic mass (not the mass #)
Period number (row) the number of energy levels outside of the nucleus
Group number (column) the second digit tells us the number of valence (outer) electrons in a Bohr atom
+Bohr model of the atom
Yep! Somebody lied when they told you 8! We’ll go into details in Unit 4. Hold your horses…
(protons & neutrons)
+Identify this atom: (assuming it is neutral p+ = e-)
NUCLEAR NOTATION
16O8
HYPHEN NOTATION
Oxygen - 16
+If we drew a Bohr model of sodium: How many protons?
How many neutrons?
How many energy levels?
How many electrons?
23Na
+Scanning Electron Microscopes http://www.youtube.com/watch?v=egPQZw0QkVw