Transcript Atomic Theory

Atomic Theory
CMS Science 4.0
Development of Atomic Models
Atom - the smallest particle of an element
Our atomic theory has grown as models
were tested and altered based on
observations and experimental results,
which led to changes and new ideas.
Ancient Greek
Democritus
Aristotle
(c460-371 BC)
(384-322 BC)
Atomos – uncuttable
Smallest possible pieces
 All matter is composed of atoms which are too
small to see.
 There is a void, or empty space, between atoms.
– Atoms are solid with no internal structure.
– Different materials have atoms of different
shapes and sizes.
John Dalton
1776 – 1844
1803
 All elements are made of atoms that cannot be
divided. Similar to hard balls.
 All atoms of the same element are exactly alike.
 Atoms cannot be changed into atoms of another
element.
 Compounds are formed by joining atoms of two or
more elements in constant proportions.
Sir John Thompson
1856 – 1940
1897
Discovered the electron, common to all elements.
Atoms have positive and negatively charged parts.
– Plum pudding (Muffin) model – negative electrons in a
positively charged ball.
Ernest Rutherford
1871 – 1937
1911
Nucleus is a tiny dense positively charged mass
Peach pit model
Protons -positively charged particles in the
nucleus
Niels Bohr 1885 - 1962
1913
Electrons are found certain distances away from the
nucleus with specific amounts or quantities of energy.
Orbits further from the nucleus have more energy.
Orbits are called energy levels or electron shells.
– Solar system model, planets orbiting the sun.
The Bohr Model
Electron cloud
1920’s
Electron’s location and movement
are related to the energy level
but their exact location is
never known
Werner Heisenberg
1901 – 1976
1925
The Heisenberg Uncertainty Principle
Electrons cannot be pinpointed at any given
moment:
Travel near the speed of light
The electron cloud is huge
James Chadwick 1891 – 1974
1932
Discovered the neutron.
Neutral – no electrical charge
Mass slightly greater than a proton
Isotopes are atoms with the same atomic number,
number of protons, but a different number of
neutrons.
When comparing Hydrogen atoms and Helium atoms,
Chadwick found that Hydrogen has one proton and Helium has two.
•When massing the atoms, H was found to have a mass of 1 and Helium
a mass of 4.
•What accounted for the extra two mass units?
2 Neutrons.
The Particles of the Atom
Particle
electron
symbol
e-
charge
-1
mass, kg
9.10953×10-31
mass, (amu)
0.000548
proton
p+
+1
1.67265×10-27
1.007276
neutron
n0
0
1.67495×10-27
1.008665
THE MASS OF THE NEUTRON IS 1839 times greater than an electron.
Composition of the Nucleus:
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nuclei are composed of "nucleons": protons and neutrons
atomic mass units: 1 amu = exactly 1/12 the mass of a carbon-12 nucleus
Modern Atomic Model
• Nucleus contains Protons (+) and Neutrons
- almost all of the mass of the atom
- tiny compared to size of the atom
1 cm
1 km
• Electrons (–) moving in a cloudlike region
around the nucleus
Modern Atomic Model
• The protons equal the electrons
number and equal but opposite charge
- neutral charge on the atom
- number of protons is the atomic number
• All atoms of an element have the same
number of protons
• Isotopes are atoms of the same element
with different numbers of neutrons
New Particles - Quarks
•
Leptons: "Lightweight" elementary particles not composed of quarks.
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Electron
Muon
Tau
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Neutrinos
•
Hadrons: "Massive" particles composed of quarks.
– Baryons: Composed of three quarks. Found in "everyday" matter.
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Proton: quarks = up + up + down
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Neutron: quarks = down + down + up
– Mesons: Composed of a quark and an antiquark. Found in cosmic rays.
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Pion: quarks = up + down
Bosons: The exchange of these "force carriers" between fundamental
particles allows the formation of Composite Particles.
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–
–
–
–
Photon - massless, no charge, electromagnetic force carrier.
Gluon - massless, no charge, strong nuclear force carrier.
W and Z - massless, no charge, weak nuclear force carriers.
Graviton - massless, no charge, gravity force carrier.
The graviton is the only force-carrying particle that has not been observed, either
directly or indirectly.