Transcript Early Atomic Theories

Early Atomic Theories
Integrated Chemistry and Physics
Democritus
• The Greek philosopher Democritus (460-370BC) was
the first person to propose the idea that matter was not
infinitely divisible.
• He believed that matter was made up of tiny individual
particles called atomos, from which the English word atom
is derived.
• Democritus believed that atoms could not be created,
destroyed, or further divided.
• Democritus also believed that matter is composed of empty
space through which atoms move.
Aristotle
• The Greek philosopher Aristotle (384-322BC) rejected
Democritus’ atomic “theory” because he did not like the idea
that atoms moved through empty space.
• He did not believe that the “nothingness” of empty space
could exist.
• Aristotle was able to gain wide acceptance for his ideas
and his denial of the existence of atoms went largely
unchallenged for 2000 years.
Dalton
• The English schoolteacher John Dalton
(1766-1844) began the development of
modern atomic theory.
• Dalton revived and revised Democritus’
ideas based upon the results of his
scientific research.
Dalton’s Atomic Theory
1.
Elements are made of tiny particles called atoms.
2.
All atoms of a given element are identical.
3.
The atoms of a given element are different from those of any
other element.
4.
Atoms of one element can combine with atoms of other elements
to form compounds. A given compound always has the same
relative numbers and types of atoms (law of constant
composition).
5.
Atoms are indivisible in chemical processes. That is, atoms are
not created nor destroyed in chemical reactions. A chemical
reaction simply changes the way the atoms are grouped together.
Dalton’s Atomic Theory
• Was Dalton’s atomic theory a huge step toward our current
atomic model of matter? YES
• Was all of Dalton’s theory accurate?
NO
• Dalton’s theory had to be revised as additional information
was learned that could not be explained by his theory.
Dalton’s Atomic Theory
You will soon learn that Dalton was wrong about:
• atoms being indivisible (they are divisible into several
subatomic particles – the proton, neutron, and electron).
• all atoms of a given element are identical (atoms of an
element may have slightly different masses – isotopes)
J.J. Thomson
• The English physicist J.J. Thomson (1856-1940)
discovered the first subatomic particle known as the
ELECTRON.
• He did this by using a cathode ray tube and he concluded
that the mass of the charged particle (the electron) was
much less than that of a Hydrogen atom, the lightest
known atom.
• The conclusion was shocking because it meant there were
particles smaller than the atom.
J.J. Thomson
• J.J. Thomson proved Dalton’s Atomic Theory incorrect.
• Dalton said that atoms were indivisible
• But, J.J. Thomson showed that atoms were divisible into
smaller subatomic particles.
Robert Millikan
• The American physicist Robert
Millikan (1868-1953) discovered the
charge of the electron.
• So good was Millikan’s
experimental setup and technique
that the charge measured almost 100
years ago is within 1% of the
currently accepted
value.
J.J. Thomson
After the electron’s charge was discovered, two questions
arose:
1) If electrons are part of all matter and they possess a
negative charge, how is it that all matter is neutral?
2) If the mass of an electron is so small, what accounts for
the rest of the mass in an atom?
J.J. Thomson attempted to answer these questions…
J.J. Thomson
• He proposed a model known as the Plum Pudding Model
of the atom, which showed a spherically shaped atom that
was positively charged with electrons evenly distributed
throughout, like raisins distributed in a plum pudding.
• The Plum Pudding was quickly proved incorrect.
Ernest Rutherford
• The New Zealand born Ernest Rutherford (1871-1937) was a
student of J.J. Thomson at Cambridge University in England.
• Rutherford came up with The Gold Foil Experiment.
• The Gold Foil Experiment was
setup to shoot a beam of alpha
particles at a piece of gold foil that
was surrounded by a zinc sulfide
screen.
Ernest Rutherford
• If the Plum Pudding Model were correct, Rutherford
would see only minor deflections as the alpha particles
passed through.
Ernest Rutherford
• Rutherford saw some of the alpha particles go straight
through the foil without any deflection, but many of the
alpha particles hit the gold foil and deflected off at large
angles.
Ernest Rutherford
• This is because the alpha particles, which are positively
charged, are hitting the dense positively charged core in the
middle of the atom and because positive repels positive,
they are being deflected.
• The Plum Pudding Model was shown to be incorrect.
Ernest Rutherford
• Rutherford finalized his experiment concluding that there
was a tiny, dense region, which he called the NUCLEUS,
centrally located within the atom that contained all of the
atom’s positive charge and almost all of the atom’s mass.
Ernest Rutherford
• His new model also showed that most of an atom consists
of electrons moving rapidly through empty space.
• The electrons are held within the atom by their attraction to
the positively charged nucleus.
• The volume of space through which the electrons move is
huge compared to the volume of the nucleus.
Ernest Rutherford
• Later, Rutherford did further research to conclude that the
positively charged particle was to be called a PROTON and
that they have a charge equal to but opposite of an
electron.
James Chadwick
• Rutherford’s student, an English
physicist named James Chadwick
(1891-1974) discovered the final
subatomic particle, the NEUTRON,
which is present in the nucleus, has
a neutral charge, and is about
equal in mass to the proton.