Download Dalton`s Atomic Theory

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Democritus- 5th Century philosopher from
the Greek city of Abdera
Student of Leucippus
Conceived the idea of the “uncuttable”...the
atom
Democritus conceived of the Void as a
vacuum, an infinite space in which moved an
infinite number of atoms that made up Being
(i.e. the physical world). These atoms are
eternal and invisible; absolutely small, so
small that their size cannot be diminished.
He claimed that there was no smallest part of
matter and that different substances were made
up of proportions of fire, air, earth, and water. As
there were of course no experimental means
available to test either view, Aristotle's prevailed
mainly because people liked his philosophy
better.
384-322 BC
John Dalton - English
1803-08 Published his “New
System of Chemical Philosophy”
whereby he championed the
atomic theory.
Gave several elements their
symbols and arranged them in
a table.
Deduced that different elements
have atoms of different mass.
Dalton's Atomic Theory
1) All matter is made of atoms. Atoms
are indivisible and indestructible.
2) All atoms of a given element are
identical in mass and properties
3) Compounds are formed by a
combination of two or more different
kinds of atoms.
4) A chemical reaction is a
rearrangement of atoms.
J. J. Thomson - English
1897- Experimented with
cathode ray tubes. Discovered
rays were made of particles
smaller than the atom. These
negatively charged particles
he called electrons.
Thomson deduced that the
atom was a positively charged
ball with lots of electrons
stuck into its surface.
Ernest Rutherford - New Zealand,
England, & Canada
Went to Cambridge to study with
J.J. Thomson. In 1898
discovered particles he called
alpha particles. By 1910, he had
determined them to be
positively charged protons
occupying a central area of the
atom. This, he postulated, was
the atomic nucleus. Such a monumental discovery marked him as
one of the greatest scientists of
any age.
Neils Bohr…Denmark
Student of Rutherford’s…1913
Perhaps I have found out a little about the
structure of atoms. Don't talk about it to anyone,
for otherwise I couldn't write to you about it so
soon. ... You understand that I may yet be wrong;
for it hasn't been worked out fully yet (but I don't
think its wrong). ... Believe me, I am eager to
finish it in a hurry, and to do so I have taken a
couple of days off from the laboratory (this is
also a secret).
Things are going rather well, for I believe I have
found out a few things; but, to be sure, I have not been
so quick to work them out as I was stupid to think. I
hope to have a little paper ready and to show it to
Rutherford before I leave, and I therefore am so busy, so
busy.
Bohr's theory that electrons existed in set orbits around
the nucleus was the key to the periodic repetition of
properties of the elements. The shells in which electrons
orbit have different quantum numbers and hold only
certain numbers of electrons -- the first shell holds no
more than 2, the second shell up to 8, the third 10, the
fourth 14.
James Chadwick - England 1932
“While working with Ernest Rutherford, Chadwick heard him
speculating on the existence of a particle with no charge which
sparked his interest. James performed experiments where he
bombarded certain light elements with alpha particles leading to
the discovery of a new type of particle, the neutron. For
this,he won the Nobel Prize for physics in 1935. James
Chadwick would die on July 14, 1974.”
Electron Cloud
Model
Since electrons are travelling so fast, it is
impossible to be able to detect where they are at
any one time. Hence, the theory that they are
constantly moving from level to level creating a
cloud effect.
So…if opposite forces attract and like
forces repel, and the nucleus of an
atom is composed of neutrons and
positively charged protons, why don’t
the protons repel each other and blow
the atom apart? What holds the protons
together?