Atomic Theory
(AKA History)
November 10, 2015
8th grade Chapter 3.1
Meme Moment
Symbols for Molecules
HBrO4
Each atom has a symbol
from the periodic table
Numbers say how many
No number = 1
Symbols can be 1 or 2 letters.
First = capital, second = lower case or none
Atomic Theory Changes
• Sometimes science
changes very quickly
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1500s alchemy
1600s alchemy + chemistry
1700s chemistry
1803 atoms!
1904 subatomic particles!
1911 it changes again!
1913 it changes again!
1926 quantum mechanics
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!
Dalton’s Model
Dalton’s Experiments Showed:
• Atoms are the smallest unit of matter
o You can’t get smaller according to his theory
o 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 ratios
o Table salt would always be 1 sodium + 1
chlorine
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 form
o Science fixes itself! We have to revise theories when new
data says we don’t have all of the facts
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
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”
Rutherford’s Experiment
Rutherford’s Experiment
• Rutherford was trying to prove
Thomson’s model
• Wanted to show that the slightlypositive 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
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
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”
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
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
Chadwick’s Model
• 8 years later...
• James Chadwick also
worked for Rutherford –
then he heard about an
experiment from Irène
and Frederic Joliot-Curie
o Something was knocking
protons out of wax using
radiation
• Chadwick proposes the
neutron – equal mass to a
proton, but uncharged
Vocabulary
Nucleus: 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
Vocabulary
Energy 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