C.6.A – Understand the experimantal design and conclusions used in the developmnet in the modern atomic theory, including Dalton’s
Postulates, Thomson’s discovery of electron properties, Rutherford nuclear atom, and Bohr’s nulcear atom; Supporting Standard.
1
STATION #1
Democritus vs. Aristotle
Not until around 460 B.C., did a Greek philosopher, Democritus, develop
the idea of atoms. He asked this question: If you break a piece of matter in
half, and then break it in half again, how many breaks will you have to
make before you can break it no further? Democritus thought that it ended
at some point, a smallest possible bit of matter. He called these basic
matter particles, atoms.
He proposed that all matter was composed of Indivisible particles called
atoms (Indivisible - Greek for uncuttable).
Unfortunately, the atomic ideas of Democritus had no lasting effects on
other Greek philosophers, including Aristotle. In fact, Aristotle dismissed
the atomic idea as worthless. He asked Democritus: “What holds the
atoms together? “Aristotle could not believe the idea that atoms moved
through empty space & he could not test his ideas to see if they were
VALID since nobody knew anything about the Scientific Method. People
considered Aristotle's opinions very important and if Aristotle thought the
atomic idea had no merit, then most other people thought the same also.
(Primates have great mimicking ability.)
Unlike early scientist who were philosophers that relied heavily on logic to
arrive at the conclusions, the scientist of the 1700’s and 1800’s relied
more on their observations to arrive at conclusions.
Nearly 2000 years passed until before scientist began studying and
considering Democritus’s ideas about the make up matter.
STATION #2
Dalton’s Atom Model
In the 1800's an English chemist, John Dalton performed experiments with
various chemicals that showed that matter, indeed, seem to consist of
elementary lumpy particles (atoms). Although he did not know about their
structure, he knew that the evidence pointed to something fundamental.
Dalton revisited and revised Democritus’s ideas based upon the results of
scientific research he conducted.
1803 - 5 Postulates of Dalton’s Atomic Theory:
(Are all of Dalton’ Theory accurate? – NO – see #2 and #3)
1.
2.
3.
4.
5.
All matter is composed of tiny particles called atoms
All atoms of an element are identical, having the same size, mass,
and chemical properties. (Later, this postulate will be proved
incorrect – isotopes of the same element have a different number
of neutrons and therefore different masses). So, all atoms of an
element are NOT identical. The number of neutrons varies from
atom to atom of the same element. The concept of isotopes will be
covered in May).
Atoms cannot be created, divided into smaller particles or
destroyed. (Dalton was wrong about being indivisible- atoms are
divisible into several subatomic particles).
Atoms combine in simple whole # ratio to form compounds
Chemical reactions are rearrangements of atoms (This proved the
proposed law of conservation of mass). Atoms of 1 element cannot
be changed into a new element during chemical reactions.
Alchemists of the time were trying to change Pb (lead) into gold
(Au).
Billiard Ball Model (1803)- John Dalton viewed the atom as a small solid
"ball" rolling for modern chemistry!
sphere. He really got the
C.6.A – Understand the experimantal design and conclusions used in the developmnet in the modern atomic theory, including Dalton’s
Postulates, Thomson’s discovery of electron properties, Rutherford nuclear atom, and Bohr’s nulcear atom; Supporting Standard.
3
STATION #3
Thomson’s Atom Model
In 1897, the English physicist J.J. Thomson proposed
that the atom was a sphere of positive electricity with
negative particles imbedded throughout after discovering
the electron, a discovery for which he was awarded the
Nobel Prize in physics in 1906.
His atom model pictured a positively charged atom
containing negatively charged electrons. His model
looked like plums stuck on the surface of a lump of
pudding analogous to plums in English plum pudding.
Thus, the Thomson proposal became popularly known as
the
plum
pudding
model
of
the
atom.
This model was not correct, but it was consistent with
the evidence at the time.
Through studying cathode ray tubes (gases in tubes carrying an electric current), he discovered
negatively charged particles. Atoms were overall neutral, so atoms must be comprised of charged
particles. He called the negatively charged particle the “corpuscle”, which later became known as
the ELECTRON. Atoms were neutral, so if the atom contains (-) particles, it must also contain (+)
particles to balance this charge. Thomson, however, never did discover this (+) particle.
It was startling to imagine a particle residing inside the atom--most people thought that the atom
was indivisible, the most fundamental unit of matter.
J.J Thomson determined the ratio of electron charge to its mass by measuring the effect of both
magnetic and electric fields on cathode ray. Thomson concluded that the mass of the charged
particle was much less than that of a hydrogen atom, the lightest known atom. So, Dalton was
wrong: Atoms were divisible into smaller subatomic particles.
“cathode ray tube”
Thomson's speculation was not unambiguously supported by his experiments. It took more
experimental work by Thomson and others to sort out the confusion. The atom is now known
to contain other particles as well. Yet Thomson's bold suggestion that cathode rays were
material constituents of atoms turned out to be correct. The rays were made up of
electrons: very small, negatively charged particles that are indeed fundamental parts of
every atom.
The accidental discovery of the cathode ray led to the invention of one of the most important
technological and social developments of the 20th century – the television.
C.6.A – Understand the experimantal design and conclusions used in the developmnet in the modern atomic theory, including Dalton’s
Postulates, Thomson’s discovery of electron properties, Rutherford nuclear atom, and Bohr’s nulcear atom; Supporting Standard.
5
STATION #4
Rutherford Nuclear Model of the Atom
In 1911, Rutherford proposed a new model of the atom. He suggested that negatively
charged electrons were distributed about a positively charged nucleus.
Rutherford designed and conducted an experiment to see if alpha particles would be
deflected if they passed through a thin foil of gold. He expected most alpha particles
to pass through the gold foil, without deflections. During the experiment however, he
noticed that some alpha particles were scattered at small angles, and some were
deflected at very large angles that were caused by the strong force of repulsion b/w
positive nucleus and positive alpha particles. He concluded that there is something
dense, positively charged, concentrated in the middle of an atom that caused those
large deflections. According to Rutherford it was the nucleus.
Rutherford concluded that the plum pudding model was incorrect because it could be
not explain the results of the gold foil experiment. Rutherford also concluded that the
atom is mostly empty space with a dense positively charged nucleus in which
nearly all the mass is concentrated.
Rutherford’s Scattering “Gold Foil”
Experiment
Nuclear Atom Model
STATION #5
Bohr’s Model of the Atom
Problem with the modern model of the Atom:
Since oppositely charged particles attract each other, why did
not the positive nucleus draw the negative electrons into it,
causing the atom to collapse?
 In 1913, Neils Bohr speculated that electrons orbit (travel)
around the nucleus in seven circular orbits just as planets
circle around the Sun.
 He suggested that the electron orbits were at a fixed distance
from the nucleus and had a definite energy.
 The electron was said to travel in 7 circular fixed-energy
orbits around the nucleus that were referred to as an energy
levels.
Planetary model of the atom
C.6.A – Understand the experimantal design and conclusions used in the developmnet in the modern atomic theory, including Dalton’s
Postulates, Thomson’s discovery of electron properties, Rutherford nuclear atom, and Bohr’s nulcear atom; Supporting Standard.
7
STATION #6
The Quantum Mechanical Model of Atom
In 1926, Austrian physicist E. Schrodinger proposed the modern model for the hydrogen atom that
seemed to apply equally well to atoms of other elements – an area in which Bohr’s model failed (it
worked only hydrogen atom). In the modern model electrons are treated a waves. The model
makes no attempt to describe the electron’s path around the nucleus.
A three-dimensional region around the nucleus called an anatomic orbital describes the electron’s
probable location – a fuzzy cloud represents the probability of finding the electron.
STATION #7
Chadwick, James
Problem with the Rutherford’s model:
Why is a nucleus so dense? His model could not account for the total
mass of atom. It was always heavier than predicted.
In 1932, Rutherford’s co-worker, English physicist James Chadwick,
showed that the nucleus also contained another subatomic particle called
the neutron. A neutron has a mass nearly equal to that of a proton, but it
carries no electrical charge. The neutrons act as “pillows” between
protons preventing them from repelling one another ( same charges cause
repulsion).