atomic theory (aka history) november 10, 2015 8 th grade chapter 3.1
TRANSCRIPT
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Atomic Theory (AKA History)
November 10, 20158th grade Chapter 3.1
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Meme Moment
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Symbols for Molecules
HBrO4
Each atom has a symbolfrom the periodic table Symbols can be 1 or 2 letters.
First = capital, second = lower case or none
Numbers say how manyNo number = 1
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Atomic Theory Changes
• Sometimes science changes very quickly
• 1500s alchemy• 1600s alchemy + chemistry• 1700s chemistry• 1803 atoms!• 1904 subatomic
particles!• 1911 it changes again!• 1913 it changes again!• 1926 quantum mechanics
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Atoms• Atom: smallest unit of matter that can still be
considered an element
• Atomic theory uses different models of atoms to explain the trends we see
• Atomic theory changed as new experiments explained more and more about atoms
• Now we know that atoms aren’t the smallest unit of matter – we have subatomic particles too!
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Dalton’s ModelDalton’s Experiments Showed:• Atoms are the smallest unit of matter
o You can’t get smaller according to his theoryo Hard spheres
• Atoms of the same element are exactly alike. Different elements have different atoms
• Atoms can never change• Compounds form when atoms combine
in set ratioso Table salt would always be 1 sodium + 1
chlorine
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100 Years of Dalton• Democritus & Aristotle used the idea of atoms to
describe the universe – Dalton used science to prove it!
• For 100 years or so, this is unchanging fact
• Then Thomson comes along in1897 and tries a new experiment... In 1904, atomic theory changes to a more detailed formo Science fixes itself! We have to revise theories when
new data says we don’t have all of the facts
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Thomson’s Experiment
Lights up when hit by beam
When electric charge was introduced,the beam bent. Opposite charges attract, so there must be a negative charge to this beam
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Thomson’s Model• With his experiments, Thomson
discovered electrons• Electrons had a negative charge
if they were bent towards the positive charge
• Thomson’s electrons were smaller than atoms and found inside atoms
• Already knew atoms had a neutral charge, so there must be a positive part to balance it out
• “Seeds in a watermelon” or “chocolate chip cookie”
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Rutherford’s Experiment
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Rutherford’s Experiment
• Rutherford was trying to prove Thomson’s model
• Wanted to show that the slightly-positive gold foil could slightly bend the path of the mostly-positive atoms – he failed
• Some of the particles bounced• There must be something hard and
positive in the center of the atom in order to bounce like that – the nucleus!
• The nucleus contains protons• Thomson’s model only lasts for 7
years
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Bohr’s Theory• Revisited an old experiment from 1800s, found new
interpretation
Background Info:• Different elements emit light in very specific places• The light is from electrons “jumping” and releasing
energy• Red = low energy, blue = high energy
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Bohr’s Model• If you only got certain
colors, you could only be using certain energies, so electrons needed to be in certain places – orbits
• Was Rutherford’s student – changes his model in 2 years
• “Rings on a tree” or “planetary orbits”
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Schrodinger & the Cloud Model
• This is very simplified in your books because the math looks like this:
• When the math stops using numbers, it’s a problem
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Schrodinger/Cloud • 13 years later...• Electrons aren’t in set
places – they are probably in places
• Mapping where the electrons most likely are gives clouds around Bohr’s rings
• In bigger atoms, some of the clouds are more complicated than spheres
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Chadwick’s Model• 8 years later...• James Chadwick also
worked for Rutherford – then he heard about an experiment from Irène and Frederic Joliot-Curieo Something was knocking
protons out of wax using radiation
• Chadwick proposes the neutron – equal mass to a proton, but uncharged
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VocabularyNucleus: the small, heavy center of an atom where protons and neutrons are located
Proton: small, positively charged particle found in the nucleus. Each element has a unique number of protons
Neutron: small, neutral particle found in the nucleus.
Electron: small, negatively charged particle that moves around outside the nucleus
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VocabularyEnergy level: roughly energy of an electron/where an electron is located
Atomic number: the number of protons.
Mass number: the number of protons + neutrons. This is often different from the atomic mass shown on the periodic table (which is an average of the mass numbers for different isotopes)
Isotope: atom with the same number of protons, but different number of neutrons compared to other atoms of the same element. E.g. Carbon-13 vs Carbon-12