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The Atom: History Recap
Democritus: The indivisible atom
• Matter is composed of empty space through which the atoms move
• The differing properties of matter are due to the different sizes, shapes and movement of the atoms.
Solid, homogeneous, indestructible and
indivisible (400 BC)
• Changes in matter are due to changes in the arrangements of the atoms and not from changes in the atoms themselves.
(atomos = “uncuttable”)
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Dalton’s atomic theory: particulate nature of matter
• Indivisible atom• All atoms of an element are identical (same size,
mass and properties) and unique (not like any other element’s atoms)
• Atoms combine in simple whole number ratios to form compounds.
• Chemical reactions cause atoms to be separated, combined or rearranged.
2 H2 1 O2 2 H2O
+
(1803)
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• 1897-Measures the mass to charge ratio of the electron and discovers that it is smaller than a hydrogen atom. The ‘indivisible’ atom is no longer indivisible.
• 1904-Proposes that the atom consisted of two parts…a large positively charged mass with electron ‘raisins’ mixed in with it.
J.J. Thomson: The ‘plum pudding model’
Plum-pudding model of the atom
(1904)
• 1909-Robert Millikan determines an electron’s charge (-1) and uses Thompson’s mass to charge ratio to calculate the mass of an electron:9.1 x 10-28 g
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Ernest Rutherford: gold-foil experiment (1911)
Experiment: Bombard thin gold foil with alpha particles (nuclei of helium atoms) and measure the amount of deflection.
Hypothesis: If Thomson’s model of the atom is valid, then a positive particle will be able to pass through an atom with little deflection.
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Ernest Rutherford: gold-foil experiment (1911)
•Most of the atom consisted of empty space.
•All of the positive charge and virtually all of the atom’s mass is contained in a dense region in the center called the nucleus.
•The electrons move rapidly through empty space outside of the nucleus.
The nuclear atom
•1920-Rutherford refines model: the nucleus contains positively charged particles called protons (+1 charge).
•1932-Third subatomic particle, the neutron (0 charge) discovered by James Chadwick.
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Niels Bohr: planetary model of the atom (1913)
Bohr planetary model
• Electrons travel along fixed orbits around the nucleus.
• Each orbital has a specific energy value.
• The energy of the orbits increases with orbital diameter.
• Electrons only move to a higher energy orbital after absorbing a quanta (amount)of energy equivalent to the difference in energy values of the two orbitals.
• When electrons ‘fall’ back down into a lower energy orbital, they release the quanta of energy.
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Schrödinger: Electron cloud model (1926)
• In the quantum mechanical model of atomic structure, electrons do not travel along fixed paths like planets around the sun.
• Instead, electrons can be found most frequently in certain areas around the nucleus.
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y
x
z
Quantum mechanical model: 90% probability of finding electron within 3D orbital with a maximum energy defined by the principal quantum number, n. (n = the row number on the periodic table)
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3d orbitals
10 e-
n = 3 9 sublevels 18 electrons n = 3 9 sublevels 18 electrons
3s2 e-
3p6 e-
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Write at least one paragraph to answer the following question:
How does the development of the atomic model (Democritus to Schrödinger) exemplify application of the scientific method?
DemocritusDalton ThomsonRutherfordBohrSchrodinger400 BC 1803 1904 1911 1913 1926
Homework tonight: Writing Assignment #2