Atomic Theory
Chemistry 6(A)
Atomic Theory
Lesson Objectives
• Understand the experimental design and conclusions used
in the development of modern atomic theory including
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Dalton’s Postulates
Thomson’s discovery of electron properties
Rutherford’s nuclear atom
Bohr’s nuclear atom
Development of Atomic Theory
• First credible atomic theory introduced by John Dalton in
1808
• Dalton studied the ratio in which elements combine when
reacted with each other
• Based theory on experimental results gathered by other
scientists
Dalton’s Experimental Theory
• Dalton’s Postulates
– Elements are made of small and indivisible particles called atoms
– Atoms of a given element are identical to one another but differ
from atoms of other elements
– Chemical reactions occur when atoms are combined, separated,
or rearranged in simplified whole-number ratios
– Atoms are neither created nor destroyed in chemical reactions
• Shortcomings of Dalton’s Postulates
– Atoms of an element can have masses that differ slightly
– Atoms can be divided into subatomic particles
Atoms and Electricity
• Scientists noticed that
some substances
decomposed when
exposed to electrical
current
– Elements are held
together by electrical
forces
• Scientists discovered the
cathode ray
Cathode Ray Tube
Discovery of the Electron
• Rays in a cathode tube were
attracted to a metal plate that
had a positive charge
– Cathode ray – stream of
negatively charged particles
• Thomson calculated the
electric charge to mass ratio
of the particles in cathode
rays
– Mass of these particles
was much less than the
atom with the least mass
• Electron – negatively
charged subatomic particle
Cathode Ray Tube
Thomson’s Atomic Model
• Thomson formulated the
“Plum Pudding” model
– Atoms are made of a
positively charged mass
and electrons are scattered
throughout the mass, like
plums mixed in a pudding
• Failed to correctly
establish the structure of
the atom
Plum Pudding Model
Rutherford’s Gold Foil Experiment
• Bombarded a thin foil of gold with alpha particles
– Alpha particles have a +2 charge
• Discovery of nucleus
– Nucleus – positively charged, dense center of an atom
Rutherford’s Nuclear Atom
• Rutherford estimated that diameter of the nucleus was
only 1/10000 of the diameter of the atom
• Nucleus accounts for almost all of an atom’s mass
– Nucleus contains protons and neutrons
• Proton – positively charged subatomic particle
• Neutron – neutrally charged subatomic particle
• Rutherford’s nuclear atom showed a positive nucleus
orbited by electrons
By en:User:Cburnett [GFDL]
Bohr’s Nuclear Atom
• Following discovery of the nucleus, Bohr proposed his
own atomic theory
– Tried to explain why the emission spectrum of the hydrogen
atom included only some light frequencies
– Proposed a diagram that explained electron movement and
energy relations
Hydrogen gas
discharge tube
Silt
Prism
Detecting
Screen
Bohr’s Nuclear Atom
• Proposed that electrons
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Photon
Are arranged in circular orbits around the nucleus e
Do not lose energy as they orbit
In each orbit have fixed amount of energy
In an orbit closest to the nucleus are in their lowest energy state
Gain and lose energy in certain, fixed amounts called quanta
May receive energy and move to a higher energy orbit
Return back to lower energy orbit when they release energy
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• Photon – particle of electromagnetic radiation that possesses a quantum
of energy
– Energy of each photon corresponds to a certain frequency
Short Comings of Bohr’s Nuclear Atom
• Did not explain emission spectrums of other elements
• Did not explain chemical properties of atoms
• Further experimentation indicated that electrons don’t
revolve in circular orbits
– Movement of electrons is much more complex
Atomic Theory
Lesson Objectives
• Understand the experimental design and conclusions used
in the development of modern atomic theory including
–
–
–
–
Dalton’s Postulates
Thomson’s discovery of electron properties
Rutherford’s nuclear atom
Bohr’s nuclear atom