Download Atomic History WebQuest

The History of the Atom
A Timeline of Thousands of Years of
Thinking about Matter
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
within them, but were surrounded by open space
Democritus’s teacher was Leucippus.
Read a paragraph about him, here.
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!
The link below is an experiment. You may do it for a
learning plan activity, or you can just watch it as part of
the Webquest:
~1700’s
From Philosophy to the Age of Reason:
Newton and the Birth of the Scientific Method
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.
–
What is the phlogiston theory?
–
Describe Antoine and Marie-Anne Paulze Lavoisier’s discovery:
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 constant composition).
Law of Definite Proportions/Law of Multiple Proportions:
Another explanation:
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.
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)
One more time, because it’s that good...
History of the Atom
Notice that large numbers of discoveries of elements
coincide with other scientific discoverieis!
Why was the electron the first
subatomic particle that was
discovered?
*Try some experiments with static
electricity!
Static Electricity Experiments
*Learn about the Crookes tube!
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.
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.
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. This links to a story of Becquerel’s discovery:
• 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
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).
• 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.
History of the Atom
What was wrong with Rutherford's model?
*The evidence that different elements burn
to produce different colored flames, along
with the work of Planck and others into a
smallest particle of energy, led to a revision
in Rutherford's model.
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.
Whose atomic models are depicted below?
What are the key characteristics of each? What
discoveries led to each new model?