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The Structure of an Atom
Chapter 3
Early Theories
• Greek Philosophers– 4 Elements
• Air
• Fire
• Wind
• Water
– Democritus• Atoms make up matter
– Aristotle• Refuted Democritus
and atom’s existence forgotten
Deomcritus
Atoms
Differences in atoms
Dalton’s Atomic Theory
• All matter is composed of extremely small particles called atoms.
• All atoms of a given element are identical.• Atoms cannot be created, divided into smaller
particles, or distroyed.• Different atoms combine in simple whole number
ratios to form compounds.• In a chemical reaction, atoms are separated,
combined or rearranged.
Dalton’s Atomic Theory
• All matter is composed of extremely small particles called atoms.
• All atoms of a given element are identical.• Atoms cannot be created, divided into smaller
particles, or destroyed. (This part proven wrong)• Different atoms combine in simple whole number
ratios to form compounds.• In a chemical reaction, atoms are separated,
combined or rearranged.
Deomcritus
Atoms
Differences in atoms
Dalton
•Atoms
•Sameness
•Created/destroyed
•Combination
•Rearragement
Subatomic Particles and the Atom
• Cathode ray tube– Stream of charged
particles (electrons).
– See pg 60 for picture
• J. J. Thompson– Used cathode ray tube
to prove existence of electron.
– Proposed “Plum Pudding Model”
Subatomic Particles and the Atom
• Plum Pudding• J. J. Thompson• Plum Pudding Model
Deomcritus
Atoms
Differences in atoms
Dalton
•Atoms
•Sameness
•Created/destroyed
•Combination
•Rearragement
Thompson
•Atoms composed of electrons
Ernest Rutherford
• Gold Foil experiment– Used to prove the
existence of a positively charged core (Nucleus)
• The results were “like firing a large artillery shell at a sheet of paper and having the shell come back and hit you!”
Ernest Rutherford• What should have
happened • What DID happened
Deomcritus
Atoms
Differences in atoms
Dalton
•Atoms
•Sameness
•Created/destroyed
•Combination
•Rearragement
Thompson
•Atoms composed of electrons
Rutherford
•Positively Charged Nucleus
Subatomic Particles and the Atom
• Chadwick– Worked with Rutherford.– Noted there was energy in the nucleus, but wasn’t the
protons.– Concluded that neutral particles must aslo exist in
nucleus.
• Mosley– Assigned atomic number to atoms
• Bohr– Designed “Bohr Model” of the atom.
Deomcritus
Atoms
Differences in atoms
Dalton
•Atoms
•Sameness
•Created/destroyed
•Combination
•Rearragement
Thompson
•Atoms composed of electrons
Rutherford
•Positively Charged Nucleus
Chadwick
•Neutrons exist in Nucleus
Subatomic Particles and the Atom
• Three main particles: (see chart on pg 97)– Proton
• Positive• In nucleus
– Neutrons• Neutral• In nucleus
– Electrons• Negative• Orbiting the nucleus (not inside)
Atomic Differences
• Atomic Number– Distinguishes one atom
from another.
– = #p+ (& #e- for a neutral atom)
• Isotopes– Differ in mass number
• Mass # = #p+ + #no
– Change is in # no
• Atomic Mass Unit
– Defined as 1/12 mass of a Carbon-12 atom
• p+ = 1 amu
• no = 1 amu
• e- = 0 amu
Atomic Differences
• Mass on the periodic table is based on the percent abundance of each isotope of that element.
• Therefore it is a decimal.• Ex: Chlorine exist naturally as Cl-37 and Cl-
35. Cl-37 is about 75% and Cl-35 is about 25%. So, (37 X .75) + (35 X .25) = 36.5 amu.
• See Ex problem on pg 103 for more ideas.
Questions?
• Ask now, or forever come in the morning for them!
