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Introduction to Atoms
Chapter 14
Section 1
History of Atom
All atoms share the same basic structure
 During past 200 years, scientists have
proposed different models
Dalton’s Model
Based on experiments, Dalton developed
a theory of structure of matter
 4 main concepts:
All matter is composed of tiny, indivisible
particles called atoms
 Atoms of each element are exactly alike
 Atoms of different elements have different
 Atoms of different elements can join to form
Dalton’s Model
Thomson’s Model
End of 1800s
 Thomson discovered that atoms were not
simple, solid spheres
 Atoms contained subatomic particles
Very small, negatively charged
 Called them electrons
Thomson’s Model
Also knew that atoms were electrically
Must contain enough positive charge to
balance negative charge of electrons
Developed model where electrons were
stuck into a positively charged sphere
Like chocolate chips in cookie dough
Thomson’s Model
Rutherford’s Model
By early 1900s, scientists knew that
positive charge of atom comes from
subatomic particles called protons
 1911—Rutherford begins to test theory
 His experiments led him to believe that
protons are concentrated in a small area
at center of atom
Called this area the nucleus
Rutherford’s Model
Rutherford’s model describes an atom as
mostly empty space, with a center nucleus
that contains nearly all the mass
Like the pit in a peach
Bohr’s Model
Modified Rutherford’s model in 1913
 Proposed that each electron has a certain
amount of energy
Helped electron move around nucleus
Electrons move around nucleus in region
called energy levels
 Energy levels surround nucleus in rings,
like layers of onion
Bohr’s Model
Has been called planetary model
Energy levels occupied by electrons are like
orbits of planets at different distances from the
sun (nucleus)
Electron Cloud Model
Model accepted today
 Electrons dart around in an energy level
 Rapid, random motion creates a “cloud” of
negative charge around nucleus
 Electron cloud gives atom its size and
Electron Cloud Model
Modern Day Atomic Structure
For any element:
 Number of Protons = Atomic Number
 Number of Electrons = Number of Protons
= Atomic Number
 Number of Neutrons = Mass Number Atomic Number
Example of Krypton
For krypton:
 Number of Protons = Atomic Number = 36
 Number of Electrons = Number of Protons
= Atomic Number = 36
 Number of Neutrons = Mass Number Atomic Number = 84 - 36 = 48