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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
masses
 Atoms of different elements can join to form
compounds

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
neutral


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
shape

Electron Cloud Model
Modern Day Atomic Structure
ELEMENTS INFORMATION
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