Transcript Thomson`s Model of the Atom

Atomic Structure
Chapter 4.1
The Nature of Scientific Models
o Tend to change over time:
Problems are discovered and explored
• New information becomes available
• New materials become available
• New technology becomes available
o Older models are often modified
o Sometimes old models totally discarded because
proved to be just plain wrong
o Studying the structure of
atoms is a little like
studying wind.
o Because you cannot see air,
you must use indirect
evidence to tell the
direction of the wind.
o Atoms pose a similar
problem because they are
extremely small.
Even with a microscope,
scientists cannot see the
structure of an atom
with “visible” light.
Ancient Greek Models of Atoms
o Cut a piece of aluminum foil in half 
• Two smaller pieces, same shiny, flexible
substance.
o Cut the pieces again, and again.
• Can you keep dividing the aluminum
pieces forever?
o Greek philosophers debated a similar
question about 2500 years ago.
Ancient Greek Models of Atoms
Democritus (Greek philosopher circa 400 BCE):
Posits matter is extremely small particles
that can not be divided.
o No hard physical proof, logical deduction
only.
o Called these particles “atoms” from the
Greek word atomos, which means “uncut”
or “indivisible.”
Aristotle (Greek philosopher circa 350 BCE)
 Posits everything made from four elements:
earth, air, fire, water.
o No hard physical proof, logical deduction only.
o Did not think there was a limit
to the division of matter.
• For centuries people accepted
Aristotle’s views.
• In 1800s, scientists had
experimental data to
support an atomic model.
John Dalton’s Evidence for Atoms
1. Dalton studied the behavior of gasses in air.
• Based on the way gasses exert pressure, Dalton
concluded (correctly) that:
A gas consists of individual particles.
2. Dalton measured masses of elements that
combine when compounds form.
Ratio of element masses was always the same
in each compound.
• In other words, compounds have a fixed
composition.
Dalton’s Atomic Theory
Example of element ratio:
When magnesium burns, it
combines with oxygen.
 Ratio of the mass of
magnesium to the mass of
oxygen is always about 3 : 2
 Magnesium dioxide has a
fixed composition.
Dalton’s Atomic Theory 1803
1. All elements composed of atoms
• atoms are tiny solid spheres
2. All atoms of same element have same
mass
• Atoms of different elements have different
masses.
3. Compounds contain more than one
element.
4. In a compound, atoms of different
elements always combine in fixed ratio.
Dalton’s Atomic Theory
Dalton made wooden spheres as a model to
represent the atoms of different elements.
• A tiny, solid sphere with a different mass
represents each type of atom.
Dalton’s Atomic Theory
• Theories must explain data from many
experiments.
Because Dalton’s atomic theory met that goal, the
theory became widely accepted.
• We now know not all of Dalton’s ideas about
atoms were completely correct.
Over time, scientists learned more and revised
the theory to take into account new discoveries.
J.J. Thomson
o First to find evidence that atoms are made
of even smaller particles.
Thomson’s Model of the Atom:
o When some materials are rubbed, they gain
the ability to attract or repel other
materials.
 Such materials are said to have either a
positive or a negative electric charge.
•Objects with like charges repel, or push apart.
•Objects with opposite charges attract, or pull
together.
Thomson’s Model of the Atom
o Amber is the hardened
form of sap, a sticky,
viscous liquid that
protects trees from
insects and disease.
If amber is rubbed with
wool, it becomes
charged and can attract
a feather.
Thomson’s Experiments
In his experiments, J. J. Thomson used a sealed tube
containing a very small amount of gas.
Sealed tube
filled with gas at Glowing beam
low pressure
Positive plate
Metal disk
Metal disk
Source of
electric current
Negative plate
Metal disk
Source of
electric current
When the current was turned on, the disks
became charged, and a glowing beam appeared
in the tube.
•Observed:
o Beam repelled by the negatively charged plate
o Beam attracted by the positively charged plate.
•Hypothesized:
o Particle interaction with air in the tube caused the air to
glow.
o Beam was a stream of charged particles
Evidence for Subatomic Particles
o Concluded particles in beam had negative charge
attracted to positive plate
o Hypothesized particles came from inside atoms
because:
For all different metals Thomson used for the disk,
the particles were same.
Particles had very, very low mass
~ 1/2000 the mass of a hydrogen atom, the lightest
atom.
o Thomson revised Dalton’s model to account
for subatomic particles.
 Dalton: atoms are tiny solid spheres
Thomson’s Model of the Atom (~1897)
o Overall atom is neutral:
has a positively charged bulk
negative charges evenly scattered throughout
Thomson’s Model of the Atom
• Thomson’s model was
called the “plum pudding”
model. Today, it might be
called the “chocolate chip
ice cream” model.
 The chips represent negatively
charged particles, which are
spread evenly through a mass
of positively charged matter—
the vanilla ice cream.
Rutherford’s Atomic Experiment
o Ernest Rutherford designed an experiment to find
out what happens to alpha particles when they
pass through a thin sheet of gold.
 Alpha particles: fast-moving, positively charged
particles.
o Rutherford hypothesized (based on Thomson’s model):
 Mass and charge at any location in the gold would be
too small to change the path of an alpha particle.
 He predicted that most particles would travel in a
straight path
The Gold Foil Experiment
 Particles light up screen
when they hit it
Deflected
particle
Undeflected
particle
Expected
Alpha
particles
Gold atoms
Slit
Observed
Beam of alpha
particles
Alpha
particles
Screen
Source of
alpha particles
Nucleus
Rutherford’s Atomic Theory
Discovery of the Nucleus
o The alpha particles whose paths were deflected
must have come close to another charged object.
The closer they came, the greater the deflection.
o However, many alpha particles passed through
the gold without being deflected. These particles
did not pass close to a charged object.
Rutherford’s Atomic Theory
o Thomson’s model did not explain all of the
evidence from Rutherford's experiment.
Rutherford proposed a new model:
• The positive charge of an atom is not evenly
spread throughout the atom.
• Positive charge is concentrated in a very small,
central area.
• The nucleus of the atom is a dense, positively
charged mass located in the center of the atom.
Rutherford’s Atomic Theory
o The Houston
Astrodome: 9 acres and
60,000 seats.
 If the stadium were a
model for an atom, a
marble could represent
its nucleus.
 Total volume of an atom
is about a trillion (1012)
times the volume of its
nucleus.
Scientists and the Atom
 Max Planck - In 1900, he produced the idea that radiation is
emitted in discrete quantities which he later called quanta.
 Albert Einstein – In 1905, building on Planck's ideas he published
the idea that the "quanta" was a bundle of light that behaved like a
particle.
 Ernest Rutherford – In 1911, working with his colleagues Hans
Geiger and Ernest Marsden, Rutherford became the first individual
to hypothesize that the center of the atom was small, dense, and
positively charged. He designated this area as the nucleus.
Scientists and the Atom
 Niels Bohr: In 1913, he proposed a model of the atom
with electrons orbiting the nucleus similar to the
planets revolving around the sun. He determined
that orbits of electrons depend on their energy, and
that electrons can jump from one energy level to
another; energy travels in discrete quantities.
 James Chadwick: In 1921, while working
with E.S. Bieler, he proposed that the
nucleus was held together by a strong
force. In 1932, he proposed that the
nucleus contained both neutrons and
protons.
 Louis de Broglie: In 1924, he proposed that electrons
could behave as waves under some conditions. This
helped scientists determine that the atom didn't
behave like the solar system because electrons do not
move in regular orbits.
Scientists and the Atom
 Erwin Schrodinger: In 1926, he developed the basic equation of
quantum mechanics building on de Broglie's concept that electrons
behave like waves in some situations.
 Werner Heisenberg: In 1927, he suggested that it is impossible to
determine the position and velocity of an electron at the same time;
this concept came to be known as the uncertainty principle.
 Max Born: In 1927, working with Heisenberg, Born modified
Schrodinger's equation of quantum mechanics. His idea enabled
scientists to develop a model of an atom displaying the nucleus
surrounded by electrons at varying locations when they are in diverse
energy states.
Summary
1. Democritus (400 BCE) – matter made of atoms.
2. John Dalton (1803) – proof for atoms.
3. J.J. Thomson (1897) – discovered electrons; plum
pudding model  positive sphere with electrons
embedded in it.
4. Ernest Rutherford (1911) – discovered nucleus;
proposed that electrons orbited a nucleus that had
almost all the mass.
5. Niels Bohr (1913) – electrons orbit in fixed paths.
6. Werner Heisenberg (1926) – Electrons move about
the nucleus in a cloud.
7. James Chadwick (1932) – nucleus contained
neutrons as well as protons.
Assessment Questions
1. The ancient Greek philosopher Democritus
coined what word for a tiny piece of matter
that cannot be divided?
a) Element
b) Electron
c) Atom
d) Molecule
Assessment Questions
2. Dalton’s theory (~1800; based on behavior of
gasses) included all but one of the
following points. Which is not from
Dalton?
a) All elements are composed of atoms.
b) Most of an atom’s mass is in its nucleus.
c) Compounds contain atoms of more than one
element.
d) In a specific compound, atoms of different
elements always combine in the same way.
Assessment Questions
3. J. J. Thomson’s gas tube electrical
experiments (~1897) provided first evidence
of:
a) Atoms
b) Nucleus
c) Subatomic particles
d) Elements
Assessment Questions
4. Rutherford’s gold foil experiment (~1914)
provided evidence for one of these:
a) Negative and positive charges are spread
evenly throughout an atom.
b) Alpha particles have a positive charge.
c) Gold is not as dense as previously thought.
d) There is a dense, positively charged mass in the
center of an atom.