Download The History of the Atom

The History of the Atom
A Timeline of Thousands of Years of
Thinking about Matter
http://www.youtube.com/watch?v=DYW50F42ss8
Leucippus and Democritus
Democritus’s universe: theorized that the two fundamental
and oppositely characterized constituents of the natural
world are indivisible bodies—atoms—and void or space
atoms move about in the void (empty space), collide, attach to others
to form compounds.
He believed atoms contained…
NO OPEN SPACE
Atom as
solid matter
History of the Atom
• 430 B.C.
– Leucippus of Miletus and Democritus of Abdera
• They debated whether a substance could be split
indefinitely (continuous matter), or if eventually there
would be a point when you could no longer break it
down (discontinuous matter).
• They theorized that the universe was made up of void
(space) and small indivisible ATOMS.
• This is the start of ATOMISM.
• 400 B.C.
– Aristotle and Plato
• Rejected Atomism – believed in Fire, Earth, Air, and
Water as the 4 elements
• This belief dominated science for over 2000 years!
History of the Atom
1770’s
– Lavoisier proposed that in ordinary chemical reactions,
matter cannot be created or destroyed (law of
conservation of mass). He also changed the phlogiston
theory to a modern theory of combustion.
1799
– Proust’s observation of the fact that specific
substances always contain elements in the same ratio
by mass led to the law of definite proportions.
Law of Definite Proportions/Law of Multiple Proportions:
http://www.youtube.com/watch?v=3dWdMqZ2UOU
History of the Atom
• 1803
– Dalton, known as the “Father of Modern Atomic
Theory,” gathered experimental evidence supporting
and explaining the theories of Lavoisier and Proust; his
was the first atomic theory based on experimental
evidence.
– Dalton’s atomic theory:
– (1) All matter is composed of atoms that are indivisible.
– (2) Atoms of the same element are identical.
– (3) Atoms of different elements are dissimilar.
– (4) Atoms of the same element can unite in more than
one ratio with another element to form more than one
compound. Atoms can unite with other atoms in simple
numerical ratios to form compounds (law of multiple
proportions).
History of the Atom
John Dalton used simple symbols to
represent elements and because he
was colorblind, he chose to use
black and white.
Element table to the right, graphic
representation of Dalton’s atomic
theory below
History of the Atom
1811
Avogadro hypothesized that equal volumes of gases, at
the same temperature and pressure, have the same
number of molecules.
http://www.youtube.com/watch?v=13WUqWd_Yk8
1865
Mendeleev arranged elements into 7 groups with similar
properties. He discovered that the properties of
elements “were periodic functions of their atomic
weights.” This became known as the Periodic Law.

History of the Atom
History of the Atom
Mendeleev's Periodic Table (1871)
http://www.youtube.com/watch?v=kuQ0Um4Wcz0
History of the Atom
History of the Atom
• 1870’s
– Crookes experiments were some of the first evidence
that electrons exist. He discovered “Cathode rays” had
the following properties: travel in straight lines from
the cathode; cause glass to fluoresce; impart a negative
charge to objects they strike; are deflected by magnets
to suggest a negative charge; cause pinwheels in their
path to spin indicating they have mass. His
experiments were a model for other scientists to use in
advancing the knowledge about electrons.
http://www.youtube.com/watch?v=u_rljVre-G4
History of the Atom
• 1896
– Becquerel found that matter containing uranium
exposes sealed photographic film. This led to the
discovery that rays are given off by uranium and
radium.
• 1898
– Marie and Pierre Curie discovered radium and its
ability to give off rays. They called the spontaneous
decay of these elements “radioactivity.”
Radioactivity
Alpha Particles: consist of 2 protons and 2
neutrons (same as the helium nucleus) and
are emitted by a radioactive source. They
are large, heavy and positively charged.
Beta Particles: high-energy, high-speed
electrons or positrons emitted by a
radioactive source. They are much
smaller and lighter than Alpha Particles
and are negatively charged.
Gamma Radiation: electro magnetic
radiation of very high frequency produced
by sub-atomic particle interactions. It is
energy, so it has no mass or charge.
History of the Atom
• 1897-1904
– J.J. Thomson, Thomson discovered the electron, the
first known particle that is smaller than the atom, and
experimentally determined its charge-mass ratio by
deflecting them by magnetic and electric fields. He also
showed that canal rays consist of positively-charged
particles and calculated the mass of the proton to be at
least a thousand times that of the electron. He also
discovered isotopes by using a type of mass
spectrometer. Thomson developed the plum-pudding
model of atoms.
http://www.youtube.com/watch?v=JwdGFZA3WOs&feature=related
http://www.youtube.com/watch?v=IdTxGJjA4Jw&feature=related
History of the Atom
1900
- Planck introduced the idea that (electromagnetic) energy is
radiated in small packets called quanta.
• 1911
– Millikan used an oil drop experiment to determine the charge on an
electron (1.602 x 10-19 coulomb) and then used Thomson’s e/m value to
calculate the mass of an electron (9.11 x 10-28 gram).
http://www.youtube.com/watch?v=XMfYHag7Liw
• 1911
Rutherford predicted the existence of the neutron
and demonstrated that the nucleus is a small, positively
charged heavy core in an atom, which consists mostly
of space. (Gold foil experiments) Suggested the name
proton for the fundamental positively charged particle,
the nucleus of the hydrogen atom.
http://www.youtube.com/watch?v=5pZj0u_XMbc
History of the Atom
History of the Atom
•1922
•Bohr used and modified Rutherford’s model of the atom. Bohr
originated the idea that electrons travel in a definite energy level
around the nucleus with no loss of energy from their motion. He
applied Planck’s idea of small packets or quanta of energy to his
electron model.
•Electrons in energy levels further from the nucleus have greater energy.
•An electron can give off a photon of energy equivalent to the difference in
energy levels if it falls from one energy level to a lower one.
•An electron cannot fall to a lower energy level if it is full (an energy level
can hold only a certain number of electron.)
•Normally every atom is in its ground state, meaning the lowest energy
levels are full and no electron can lose energy.
•Atoms can absorb energy from an outside source and an electron can jump
to a higher energy level, an excited state. It rapidly falls back and energy is
emitted.
History of the Atom
The Hydrogen Atom
History of the Atom
• Schrodinger, 1930 – Viewed electrons as
continuous clouds and introduced “wave
mechanics” as a mathematical model of the atom.
• Chadwick, 1932 – Using alpha particles
Chadwick discovered a neutral atomic particle
with a mass close to a proton. He is credited with
the discovery of the neutron.