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Chapter 4
Atom
• Smallest particle of an element that retains its
chemical identity
• Atoms of the same element cannot be
separated chemically because they share
identical chemical properties
Daltons Atomic Theory
• Dalton have four postulates (characteristics) of
atoms
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First postulate
Second postulate
Third postulate
Fourth postulate
First Postulate
• All elements are composed of tiny indivisible
particles called atoms
– Atoms cannot be divided into smaller pieces
Second Postulate
• Atoms of the same element are identical.
– Atoms of the same element have identical
chemical and physical properties. This is why they
cannot be separated from one another by
chemical means
Third Postulate
• Atoms of different elements can physically mix
together or can chemically combine in simple
whole number ratios to form compounds
– Remember when elements mix they do not have
to form compounds
Fourth Postulate
• Chemical reactions occur when atoms are
separated, joined, or rearranged. Atoms,
however, never change during a chemical
reactions
Revision of Daltons First Postulate
• Daltons first postulate was later revised to
read
– Atoms cannot be divided by chemical means
• No chemical reaction can divide atoms.
However, nuclear weapons, and nuclear
power have shown how atoms can be divided
by nuclear processes
Subatomic particles
• Three sub atomic particles exist
• Protons
• Electrons
• Neutrons
Protons
• Discovered by Eugen Goldstein
• Positively charged particle found inside the
nucleus of an atom
• Protons have a mass of approximately 1 amu
Neutrons
• Neutral particles (carry no charge)
• Found in the nucleus of an atom
• Neutrons have a mass of approximately 1 amu
Electrons
• Discovered by J. J. Thompson with the Cathode
Ray Tube experiment
• Carry a negative charge
• Found outside the nucleus of the atom
• Have very little mass (approximately 1/1840 the
mass of a proton)
Cathode Ray Tube Experiment
• Passed a “beam of electricity” called a cathode
ray through a gas in a cathode ray tube
• The beam responded to magnetic and electric
forces which indicates it was charged particles
• Those particles were later called electrons
Cathode Ray Tube Experiment
• Because the cathode ray tube experiment
could be repeated with many types of anodes
and cathodes it was found that electrons
resided in all materials.
• Hence all elements contained electrons
Rutherford Gold Foil Experiment
• Discovered atoms had a nucleus
• Fired alpha particles into gold foil
• Results
Results
• Most particles went straight through the gold
foil
– Atoms are mostly space
• Some particles bounced back
– Atoms have dense tightly packed core