Download Aristotle vs. Democritus: Early Views About Matter

Development of Atomic
Theory
The Greek Philosopher
Democritus
 Philosophers were
great thinkers, but
not true scientists.
 Democritus lived
from 460-370 BC.
The Greek Philosopher
Democritus
 Developed the
“discontinuous” idea of
matter with his teacher,
Leucippus.
 If we keep splitting matter
in half, we eventually
reach a particle that can
not be broken down
further. Atomos is Greek
for “uncuttable” or
“indivisible”.
 The idea of an atom
implies that matter
contains a lot of empty
space!
The Greek Philosopher
Aristotle
 Aristotle was uncomfortable
with the idea that matter
contains so much empty space.
 Believed that matter could be
continuously divided without
end (the “continuous” idea of
matter).
 There is no need for empty
space.
 There are no atoms. All matter
is made of the natural elements
(earth, water, air, fire, and
aether, the heavenly substance)
Real Science: Evidence based ideas.
John Dalton (1766-1844)
proposed his Atomic Theory
in 1803:
 1) All matter consists of
indivisible atoms.
 2) All atoms of a given
element are identical.
 3) Different elements have
different types of atoms.
Real Science: Evidence based ideas.
John Dalton’s Atomic Theory also includes:
4) Law of Multiple Proportions: Atoms always
combine in small, whole number ratios to
produce compounds. (ex. H2O and H2O2)
H2O
Why does this support the existence of atoms?
Real Science: Evidence based ideas.
John Dalton’s Atomic Theory also includes:
5) Law of Definite Composition: A given
compound ALWAYS contains the same
elements in the same proportions, by mass
(for example - H2O).
89% oxygen by mass
11% hydrogen by mass
ALWAYS!
Why does this support the existence of atoms?
Real Science: Evidence based ideas.
6) Law of Conservation of Mass: In a chemical
reaction the mass of the reactants equals
the mass of the products so mass is
conserved. (from Antoine Lavosier - 1789)
How would Dalton explain why this law is
always true for chemical reactions?
William Crookes (1848-1923)
 Invented a tube
(cathode ray
tube) that
emitted strange
charged particles.
 These particles
were later shown
to be electrons.
 The atom must
contain subatomic
particles.
Discovery of Subatomic
Structure
William Crookes (1848-1923)
 The existence of electricity implies
that atoms must contain negative
charges.
 Crooke developed the cathode ray
tube, which produces electrons.
Discovery of Subatomic
Structure
J.J. Thomson (1856-1940)
 Recognized that atoms are electrically
neutral.
 For every electron, there must be one
proton of equal (but opposite) charge.
Plum-Pudding
Model
Robert Millikan (1868-1953)
 Measured the charge
of an electron
 Used the charge/mass
ratio from Thomson to
then calculate the
mass of an electron!
 Mass of an electron is
= 1/2000 AMU
 Charge of an electron
= -1 Unit
Discovery of Subatomic
Structure
E. Rutherford (1871-1937)
 J.J. Thomson’s student.
 Performed the “gold foil” experiment.
Discovery of Subatomic
Structure
Rutherford’s Gold Foil Experiment
 Most of the positively
charged a particles
went straight through
the gold  atoms are
mostly empty space.
 Many a particles were
deflected  - charges
on outside.
 Some a particles came
straight back!  small +
charge at center.
Rutherford’s Model
The electrons orbit around the tiny, dense, positively
charged nucleus.
Rutherford placed all electrons at approximately the
same distance from the nucleus so all have the
same energy.
Discovery of the Neutron
James Chadwick (1891-1974)
 Rutherford’s co-worker.
 Knew that atoms are actually
“heavier” than they should be if
you just add the mass of the
protons and electrons together
 There must be a missing particle!
 Their neutral nature made
neutrons difficult to detect but
he did detect them by an
ingenious experiment.
 Neutrons are like glue that holds
protons together.
The discovery of the 3 subatomic
particles was complete by 1930.
Electrons and Light
 Observations of light led to the development
of the next atomic model – The Bohr Model –
the first “quantum” model.
 Light is produced by moving electrons.
Beyond Rutherford
Studies of light (which is produced by
moving electrons) soon revealed that not
all electrons in an atom have the same
energy.
The Atomic Emission Spectrum
of Hydrogen (Case in Point!)
Beyond Rutherford
Studies of light (which is produced by
moving electrons in atoms) suggested
that that not all electrons in an atom have
the same energy.
Scientists reasoned that
electrons close to the
nucleus might have
lower energies, whereas
electrons far from the
nucleus have higher
energies.
Beyond Rutherford
The studies of light revealed that electrons can have only
specific energies and only certain allowable orbits to represent
those energies. In other words, energy is “quantized” in matter
(atoms). It can only be absorbed and released in specific
amounts, not any amount!
Before the quantum model
of the atom.
The quantum model of the atom
rungs of a ladder.
Neils Bohr
Neils Bohr (1885-1962) was a Danish physicist that studied
electron behavior in atoms and how atoms absorb and
release energy in specific amounts called quanta.
He added the energy levels to
show that electrons in atoms
have only certain allowable
energy state (orbits), like
planets around the sun.
Neils Bohr
Each permitted orbits, called an energy level, can
hold only a maximum number of electrons:
1st energy level: Max of 2 electrons.
2nd energy level: Max of 8 electrons.
3rd energy level: Max of 18 electrons.
For each shell, maximum # electrons = 2n2.
Electrons fill the lowest available energy levels
before filling higher energy levels!
Neils Bohr
The Bohr model explains that electrons moving between
energy levels is what produces the atomic emission
spectrum. When matter absorbs energy, the electrons jump
out to higher energy levels. When they release the energy
(as light) they fall back down to lower energy levels!
The Modern Quantum
Mechanical Model of the Atom
 Based on the fact that electrons
exhibit significant wave properties
 Video Clip – Dr. Quantum and the
Double Slit Experiment
 Electrons cannot be located with
certainty outside of the nucleus
 Electrons do not move in predictable
paths
The Modern Quantum
Mechanical Model of the Atom
 Electrons are modeled as waves and their
positions are determined by calculating high
probability regions (orbitals) outside the
nucleus where they occur depending upon
their energy.