Transcript Slide 1

Chapter 4 Atomic Structure
Democritus
• Greek philosopher
• Tried to define matter
more than 2400 years
ago.
– Could matter be
divided into smaller
and smaller pieces
forever, or was there a
limit to the number of
times a piece of matter
could be divided?
Atomos
• He theorized that matter could not be
divided into smaller and smaller pieces
forever, eventually the smallest possible
piece would be obtained.
• He named the smallest piece of matter
atomos, meaning “indivisible.”
Atomos
 Atoms were small,
hard particles
 All made of the same
material
 Different shapes and
sizes
 Infinite in number
 Always moving and
capable of joining
together
Why was this theory ignored
and forgotten for more than
2000 years?
…something was missing
John Dalton
• Early 1800s,
English Chemist
Experimentationled to the
acceptance of the
idea of atoms
• Studied ratios of
elements in rxns.
Dalton’s Atomic Theory
1. All elements are composed of
atoms. Atoms are indivisible
and indestructible particles.
2. Atoms of the same element
are exactly alike, but different
from those of any other
element.
Dalton’s Atomic Theory
3. Compounds are formed by the
joining of atoms of two or more
elements in simple whole-number
ratios.
4. Chemical reactions occur when
atoms are separated, joined, or
rearranged. The atoms of one
element are never changed into
atoms of another element.
This theory became
one of the
foundations of
modern chemistry.
The Men behind Atomic Theory
• Their quest for
knowledge about
the nature of the
universe helped
define our views.
Exit Ticket
1. Identify two points of Dalton’s
Atomic Theory.
2. Describe the structure of the
atom, including subatomic
particles, their charge and
location.
Just how small is an atom?
100,000,000
copper atoms =
1 cm
Thomson’s Plum Pudding Model
• In 1897, the English
scientist J.J.
Thomson provided
the first hint that
an atom is made of
even smaller
particles.
• Thomson studied the passage of
an electric current through a gas.
• As the current passed through the
gas, it gave off rays of negatively
charged particles.
• Thomson concluded that the negative
charges came from within the atom.
• A particle smaller than an atom had
to exist.
• The atom was divisible!
• Thomson called the negatively
charged particles “corpuscles,” today
known as electrons.
• Since the gas was known to be neutral,
having no charge, he reasoned that
there must be positively charged
particles in the atom.
• But he could never find them.
• To test his hypothesis…
• Measured the ratio of the charge of
an electron to its mass
• Ratio was constant
• Ratio did not depend on the kind of
gas in CRT or metal electrodes used
• Therefore, electrons are part of
atoms of all elements
Rutherford’s Gold Foil Experiment
• In 1908, the English physicist Ernest
Rutherford conducted an experiment that
involved firing a stream of tiny positively
charged particles at a thin sheet of gold
foil
Most of the positively
charged “bullets”
passed right
through the gold
atoms in the sheet of
gold foil.
Some of the positively
charged “bullets,”
bounced away from
the gold sheet. He
knew that positive
charges repel
positive charges.
Rutherford reasoned that
all of an atom’s
positively charged
particles were
contained in the
nucleus. The negatively
charged particles were
scattered outside the
nucleus around the
atom’s edge.
• He concluded…the gold atoms in the
sheet were mostly open space, not a
pudding filled with a positively charged
material.
• Rutherford concluded that an atom had a
small, dense, positively charged center
that repelled his positively charged
“bullets.”
• He called the center of the atom the
nucleus.
• The nucleus is tiny compared to the atom
as a whole.
Eugen Goldstein
• In 1886 found rays traveling in the
opposite direction to that of the
cathode rays
• Composed of positive particles,
which were later named protons
• Mass is about 1840 times that of an
electron
James Chadwick
• 1932 confirmed the existence of the
neutron
• No charge
• Mass is equal to the proton’s mass
Robert A. Millikan
• Calculated the mass of the electron in
1916
• An electron carries one unit of
negative charge
• An electron’s mass is 1/1840 the mass
of a hydrogen atom
Bohr Model
• In 1913, the Danish
scientist Niels Bohr
proposed an
improvement. In his
model, he placed
each electron in a
specific energy
level.
Bohr Model
• According to Bohr’s
atomic model, electrons
move in definite orbits
around the nucleus, much
like planets circle the sun.
These orbits, or energy
levels, are located at
certain distances from the
nucleus.
The Wave Model
• Today’s atomic model
is based on the
principles of wave
mechanics.
• According to the
theory of wave
mechanics, electrons
do not move about an
atom in a definite
path, like the planets
around the sun.
MODEL
Indivisible
Greek
X
Dalton
X
Electrons
Nucleus
Thomson
X
Rutherford
X
X
Bohr
X
X
Wave
X
X
Orbit
Electron
Cloud
X
X